Where Is the Windows.old Folder and Can I Delete It?
Windows.old is the full copy of your previous install that upgrades leave behind, and it deletes itself after about ten days.
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Major Windows upgrades stash the old system in C:\Windows.old instead of deleting it. User profiles, Program Files, and the old Windows directory all sit inside, which is why the folder runs to tens of gigabytes.
Its purpose is rollback, not archiving. For roughly ten days the Go back option restores the previous install from this copy. Then the system removes it automatically and the disk space returns. Pull any forgotten files out before that deadline passes.
Where Windows stores this, by platform
C:\Windows.old
Browse it like any folder to rescue old files. Do not run programs from inside it: installed software needs reinstalling under the new system, not launching from the archive.
Frequently asked questions
How do I delete Windows.old safely?
Run Disk Cleanup as administrator and tick Previous Windows installation(s). Deleting the folder by hand fights permissions and the servicing stack. Cleanup removes it properly in one pass.
Can I undo the upgrade with it?
Only inside the rollback window through Settings > System > Recovery > Go back. After Windows.old is removed, returning means a clean install and reinstalling everything by hand.
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