How to Reinstall Windows Without Losing Data: Step-by-Step Guide

To reinstall Windows without losing your files, boot from a Windows USB, choose Custom: Install Windows only, and pick the existing Windows partition without formatting it. Windows Setup automatically moves your old Windows, Users folder, Desktop, Documents, and Downloads into a folder called C:\Windows.old that you can pull files from after the new install boots. Pick the correct boot mode (UEFI for GPT drives, Legacy/CSM for MBR drives) — a mismatch is the number-one reason Setup refuses to install onto the existing partition.

Applies to: Windows 10 (22H2) and Windows 11 (23H2, 24H2, 25H2) | Last updated: April 15, 2026

Key Takeaways

• A “reinstall without losing data” works by installing the new Windows into the existing partition without formatting. Setup moves old files into C:\Windows.old, where you can rescue them afterwards.
• Programs and installed apps are not preserved — only user files. After install, reinstall apps manually or restore them from a UniGetUI bundle saved before the reinstall.
• Check your disk’s partition style with diskpart → list disk. An asterisk under GPT = GPT; no asterisk = MBR. This determines whether you boot the installer in UEFI or Legacy mode.
• If Setup shows “Windows cannot be installed on this disk — the selected disk is of the GPT/MBR partition style”, you booted the installer in the wrong mode. Reboot, change the boot mode, and pick the matching USB entry.
• Windows.old is kept for 10 days after the reinstall, then automatically deleted. Copy what you need off within that window.

Quick Steps

1. Create a Windows installation USB using Ventoy or Rufus.
2. Check the current drive’s partition style with diskpart (GPT or MBR).
3. Enter BIOS/UEFI, set the matching boot mode (UEFI for GPT, Legacy/CSM for MBR).
4. Boot from the USB using the correct firmware path (UEFI or Legacy) from the boot menu.
5. Run Windows Setup, choose Custom: Install Windows only.
6. Select the existing Windows partition without deleting or formatting it, click Next.
7. After Setup finishes, copy your files from C:\Windows.old\Users\[YourName]\ into the new user profile.
8. When you have confirmed everything is rescued, remove Windows.old via Disk Cleanup.

When This Method Is the Right Choice

This method is for when Windows is broken to the point where you can’t recover it — bootloader corrupted, Windows update failed, system files damaged, or a ransomware infection that can’t be cleaned — but the drive itself is still readable. The new Windows installs over the old one on the same partition, and your personal files survive in Windows.old.

It is not a substitute for a real backup. Before you start, if the PC still boots long enough to copy files off, do that — external drive, cloud storage, or another internal drive. Something going wrong mid-install is rare but always possible, and the only truly safe path is to already have the data somewhere else before you touch the installer.

If the PC won’t boot at all, the process still works — you’ll skip Steps 1-2 and go straight to booting from the USB. The partition-style check can be done from the installer’s Shift+F10 command prompt.

Step 1: Check the Disk’s Partition Style (GPT vs MBR)

Booting the installer in the wrong mode for your existing drive is the single most common reason a “reinstall without losing data” fails. Check the style first.

If Windows boots, open Command Prompt as Administrator and run:

diskpart
list disk
exit

Look at the Gpt column in the output. An asterisk (*) means the disk is GPT. No asterisk means it is MBR. Remember this — it drives which boot mode you pick in Step 3.

If Windows Won’t Boot

Boot into Windows Recovery by interrupting boot three times in a row (press and hold power during the spinning dots to force shutdown, repeat twice more — on the third failed boot, Windows enters Recovery automatically). Pick Troubleshoot → Advanced options → Command Prompt, then run the same diskpart / list disk commands.

Step 2: Create a Windows Installation USB

Download the latest Windows ISO and write it to a USB drive. You have two good options:

• Ventoy — my preferred multi-ISO USB tool. Install Ventoy once, then drag ISOs onto it like regular files. See my Ventoy rescue disk guide.
• Rufus — the dedicated Windows USB installer. Writes a single ISO to a USB. See my Rufus bootable USB guide.

For the ISO itself, use my Windows 11 ISO download guide or Windows 10 ISO guide — both download straight from Microsoft.

Tip: Match the ISO to your existing install where possible. Reinstalling Windows 11 24H2 over an existing Windows 11 24H2 is smoothest. Reinstalling Windows 11 over Windows 10 is also fine, but expect the upgrade to take longer.

Step 3: Set the Correct Boot Mode in BIOS/UEFI

Reboot and press the BIOS access key as the manufacturer logo appears — typically Del, F2, F10, or Esc. Your boot screen usually displays the key briefly.

Find the Boot or Boot Options section (it’s in different places in different BIOS interfaces). Look for a setting labelled one of:

• Boot Mode — typical on MSI, Asus
• CSM (Compatibility Support Module) — typical on Gigabyte, ASRock
• Boot Mode Selection / Legacy Boot — typical on Dell, HP, Lenovo

• If your existing disk is GPT, set this to UEFI (or UEFI Only, or disable CSM).
• If your existing disk is MBR, set this to Legacy (or Legacy Only, or enable CSM).

Save and exit the BIOS (usually F10). On modern systems, most drives ship GPT with UEFI. Legacy/MBR is mostly seen on drives cloned from older Windows 7 / 8 installs.

Step 4: Boot From the USB in the Matching Mode

Press the boot menu key at startup — usually F12 on Dell/Lenovo, F11 on MSI/ASRock, F10 on HP, Esc or F9 on Acer. Your USB drive often appears twice — once labelled UEFI: [USB name] and once as [USB name] alone. Pick the entry that matches:

• GPT disk → UEFI entry
• MBR disk → Legacy entry (without the UEFI prefix)

Pick the matching entry and press Enter. Windows Setup loads.

Step 5: Install Windows Over the Existing Partition

1. Pick your language, time, and keyboard layout. Click Next.
2. Click Install Now.
3. Product key screen: click I don’t have a product key. Windows will reactivate automatically from the digital licence stored against your hardware.
4. Pick the edition that matches your previous install (Home or Pro).
5. Accept the licence terms.
6. Install type: choose Custom: Install Windows only (advanced).

On the Where do you want to install Windows? screen:

1. Click the existing Primary partition that contains your current Windows install — usually the largest one.
2. Do not click Delete or Format. Just click Next.
3. Setup warns that any existing Windows installation on this partition will be moved into a Windows.old folder. Click OK to confirm.

Common Error: Disk Partition Style Mismatch

If Setup shows one of these errors:

• “The selected disk is of the GPT partition style”
• “The selected disk has an MBR partition table”

you booted the installer in the wrong mode. Reboot, go back to the boot menu, and pick the other USB entry. Do not convert the disk between GPT and MBR — that would destroy the partition table and your data along with it.

Windows Setup now copies the new install, reboots a few times, and drops you into OOBE. Configure a new user account (or re-use the same name — Windows appends a suffix if there’s a conflict). When you land on the new desktop, you’re ready to rescue files.

Step 6: Rescue Files From C:\Windows.old

Everything from your old install now lives under C:\Windows.old\. Your personal files are under:

C:\Windows.old\Users\[YourOldUsername]\Desktop
C:\Windows.old\Users\[YourOldUsername]\Documents
C:\Windows.old\Users\[YourOldUsername]\Downloads
C:\Windows.old\Users\[YourOldUsername]\Pictures
C:\Windows.old\Users\[YourOldUsername]\Videos
C:\Windows.old\Users\[YourOldUsername]\Music

Copy Files into the New Profile

1. Open File Explorer (Win + E).
2. Navigate to C:\Windows.old\Users\ and double-click your old username.
3. If Windows prompts about permissions, click Continue — it will grant you access to the old profile’s files.
4. Open each folder (Desktop, Documents, Downloads, Pictures, Videos, Music), select the contents (Ctrl + A), copy (Ctrl + C), navigate to the matching folder in your new profile (C:\Users\[NewUsername]\), and paste (Ctrl + V).

Tip: Open two File Explorer windows side-by-side by holding Shift when you click the taskbar icon. Use Win+Left/Win+Right to snap them, then drag files across. Much faster than copy-paste for large folders.

What’s Also in Windows.old Worth Looking For

• AppData\Roaming\[AppName] — saved settings for portable apps (e.g., Thunderbird profiles, Notepad++ config).
• AppData\Local\[AppName] — app data like game saves for titles that stored progress outside the cloud.
• Favorites / Bookmarks — browser bookmarks if you did not have sync enabled.
• C:\Windows.old\Program Files\[App] — some portable apps can be copied directly back and will work.

Step 7: Reinstall Your Apps and Clean Up

This reinstall method preserves files, not installed apps. To get your software back quickly:

• Install UniGetUI and re-install your usual tools in bulk. If you had exported a bundle before the reinstall, import it now.
• Install drivers with Snappy Driver Installer Origin — it catches anything Windows Update missed.
• Debloat and apply sensible defaults with Winhance.

Delete Windows.old (Once You Are Sure)

Windows.old takes 20–40 GB of disk space. Once you have confirmed every file is rescued, delete it via Disk Cleanup (the only supported way — regular Shift + Delete will not work because of file permissions).

1. Open Start → type Disk Cleanup → launch it.
2. Pick the C: drive.
3. Click Clean up system files.
4. Tick Previous Windows installation(s). Click OK and confirm.

Windows keeps Windows.old for 10 days automatically, then deletes it for you. Use Disk Cleanup if you want the space back sooner.

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Frequently Asked Questions

Do my installed programs survive a “keep data” reinstall?

No. Only personal files in the Users folder are preserved — Windows, system settings, and installed apps are all replaced. You will need to reinstall programs afterwards. A UniGetUI bundle exported beforehand makes this painless.

What is the difference between GPT and MBR?

GPT (GUID Partition Table) is the modern partition style — required for UEFI boot, supports disks larger than 2 TB, and allows more than 4 primary partitions. MBR (Master Boot Record) is the older style from the BIOS era — limited to 2 TB and 4 primary partitions, but still used on older systems. You cannot change one to the other without destroying data, so the boot mode you pick must match what is already on the disk.

What happens if I choose the wrong boot mode?

Windows Setup refuses to install and shows an error message. No data is lost from the mismatch — you simply reboot, change the boot mode, select the matching USB entry from the boot menu, and try again. The error is Setup’s safety catch; it won’t overwrite the partition with a broken boot record.

Can I reinstall without an internet connection?

Yes. The install itself doesn’t need the internet — Windows Setup reads everything from the USB. You will need a network connection eventually for Windows Update, driver downloads, and Microsoft account sign-in (which you can skip with the BypassNRO trick — see my local account bypass guide).

How much space does Windows.old take?

Typically 20–40 GB, depending on how much was installed on the old Windows — Program Files, Windows Update cache, and per-user AppData all add up. Copy off what you need, then use Disk Cleanup’s “Previous Windows installations” option to reclaim the space. Windows automatically deletes Windows.old 10 days after the reinstall anyway.

Is this the same as Reset This PC?

Not quite. Reset This PC → Keep my files inside Windows does something similar without needing a USB, but it only works if Windows is bootable and the system image is intact. This manual method works when the system is unbootable or the Reset option is broken. For a full factory reset walkthrough, see my factory reset guide.