I need to know what is unsafe here.
That said there are plenty of articles about Disk Cleanup to remove old Windows.old folders as well as a Linux boot if you need more deletion power.
I have Windows.old, Windows.old (1) and Windows.old (2) in my computer which I want to get rid of, safely, if I could.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic