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Windows.old isn't only created when reinstalling or upgrading to a new version Windows, but also during major Windows updates.

What is stored in the folder? Only system files or user data as well? I have lost Nvidia Control Panel settings in Windows 10 Anniversary Update and again in the recent Windows 10 Creators Update, but otherwise I haven't observed any loss of personal program data.
Is deleting with the Disk Cleanup tool safe, as long as I'm not reverting to an earlier version of Windows and I'm willing to reinstall in the case of major issues?

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  • Losing the Nvidia control panel likely means that Microsoft "upgraded" your video drivers. I put that in quotes, because the driver is often not an actual upgrade. You may want to go to the Nvidia website and check if there's a better version of your driver that supports your graphics card.
    – trlkly
    May 18, 2017 at 23:37
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    Also, it's only created during "major updates" because these major updates are exactly the same as an upgrade, e.g. from Win10 1607 to 1703 uses more or less the same process as Win8.1 to Win10. That's literally how they're implemented - as a full OS upgrade.
    – Bob
    May 19, 2017 at 0:09
  • @trlkly: I don't think so, the installation date isn't recent.
    – user598527
    May 19, 2017 at 7:47

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First a warning : Windows.old is almost totally deleted after 28 days, so better hurry if you wish to recover any data (Edit: For the Windows 10=>11 upgrade this is now 10 days).

This folder is created during a major upgrade of Windows, and allows the rollback to the previous version of Windows.

Here is what Microsoft says :

A custom install is similar to a clean install, but instead of wiping the hard disk clean, your previous Windows installation is moved to a folder called Windows.old. Windows.old is an archive of your old Windows installation. It is created by setup for a number of reasons depending on how the upgrade was initiated, including recovery of personal data or to facilitate the rollback to that previous version of Windows if you performed an in place upgrade.

In particular, you will find :

  • The old registry hives in C:\Windows.old\System32\config
  • Your personal profile files in C:\Windows.old\System32\Users, which includes Cookies, Start Menu, UserData and more.

For more information see the Microsoft article :
Recover personal files from the Windows.old Folder such as Documents Pictures and Windows Live Mail Emails in Windows 10

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    "Windows.old is almost totally deleted after 28 days" Have you got a source for that? I've had windows.old directories linger around for months, maybe years. I can't speak for the integrity of everything in there, but certainly documents, program files etc. were never deleted.
    – Michael
    May 18, 2017 at 14:48
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    @Michael: The folder persists forever, but most of its contents don't. For example see this source.
    – harrymc
    May 18, 2017 at 14:58
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    What is "UserData" folder?
    – user598527
    May 18, 2017 at 17:28
  • "so better hurry if you wish to recover any data" couldn't you just rename or copy the folder to another location if you wished to keep it permanently?
    – ESR
    May 19, 2017 at 0:56
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    @EdmundReed: A backup of the folder would be one way to save it, although not all of it is really interesting.
    – harrymc
    May 19, 2017 at 5:37
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Windows.old is a folder that contains a backup version of your previous Windows.

For example, if you upgrade from Windows 7 to Windows 10, then your Windows OS will create this folder and store a backup version to help you roll back if you don't like Windows 10. In this case, you can easily roll back to Windows 7 without losing any data.

This folder can be created if you install major updates in Windows 10, such as the recent Windows 10 Windows Creators Update. The purpose is to roll back in the case incompatibility.

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    This is not entirely true. The windows.old does not contain a full backup. It only contains the files that were modified during the upgrade. Windows will reconstruct the old version by replacing the files to their rightful locations. Files that weren't modified will not be in the windows.old folder.
    – LPChip
    May 18, 2017 at 10:58

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