Hello Sphinx. Yes, you can safely delete the Windows.old folder, but only when you are confident you won't be reverting to an earlier version of Windows. The Windows.old folder is automatically generated during major updates of Windows and reinstalls of Windows and contains system files from your previous installation. It is always better to use Disk Cleanup or Storage Sense to clean folders, even if you can take ownership or permissions manually by using commands like takeown or icacls. When taking ownership or permissions of a folder owned by TrustedInstaller, you run the risk of damaging system integrity and breaking access to things like Wi-Fi, Settings, or Program Files, as Windows only owns in one place on the system at all times, and somethings just can't be modified to run access friendly again, but windows will access a cloud record to ensure solid recovery. If you modify all these folders and somewhere along the line, you break it all together and windows doesn't run and you can't access effectively the device in a stable manner, the most reliable long term method of recovery, that keeps your files, is to do a recover via cloud, keep my files, that re-installs windows so not much from your recovery is stored. Remember the most reliable way to clean up is via the built in tools that are designed to just work and avoid anything owned by TrustedInstaller or performance related to essential system files stored in "C:."
Please also refer to this article which is similar with your case and was answered by Marcin Policht a VM about Removing Windows.old folder
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Best regards,