Reclaim lost space on Windows 7

I

was horrified to see the windows 7 folder on a virtual machine had grown to 25GB.

The reason is that windows keeps copies of everything it changes when it applies updates. Yes, even though you installed SP1 and you don't need anything before that ever again, MS will happily waste your precious hard drive space keeping those useless copies of stuff.

I finally found out how to get rid of all the unnecessary backups the correct way.

DISM.exe /Online /Cleanup-Image /SpSuperseded

Run it from a terminal, you will need Admin rights.