Which Windows temp files are safe to delete

What "temp" really means

Windows and the apps you run constantly write working files they intend to throw away: installer scratch space, partial downloads, render caches, crash-time snapshots. There are two main locations:

• Your user temp folder, at %TEMP% (which expands to something like C:\Users\you\AppData\Local\Temp). This is yours and is safe to clear.
• The system temp folder, at C:\Windows\Temp. Used by Windows and services. Also safe to clear, but it needs an administrator because of its location.

So is it safe to delete them?

Yes. Nothing in a temp folder is supposed to be permanent, and Windows recreates whatever it needs. The only real catch is files that are currently in use: an app that is running may have a temp file open, and Windows will refuse to delete it while it is locked.

The careful way to clear them

The built-in Disk Cleanup tool handles some of this, and you can select the temp folder contents in Explorer and delete them. Either works, but neither gives you a preview, a record of what went, or an undo.

Sifty treats user temp and system temp as two of its junk categories. See them first:

PS C:\> sifty junk scan
  User temp files            537 MB
  Windows temp files (admin) 210 MB
  ...

Then clear, with a confirm and a skip report:

PS C:\> sifty junk clean --apply
✓ Sent 12,136 items (537 MB) to the Recycle Bin.
! 41 item(s) skipped (in use).

The system temp folder needs elevation. Run sifty --admin junk clean --apply and Windows will prompt for it. Everything is sent to the Recycle Bin, every applied deletion is written to an audit log, and sifty undo restores the last clean if you need it back.