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
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
- 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