As long as there is not anything that you need in there, then yes you can
delete it. For me I have programs that I was running on XP that didn't need
to be officially installed but I put them into my program files directory
and they are still there so for me if I had deleted the Windows Old folder I
would have not access to them anymore. Also some files will default save
your data/information/gamesaves etc to their own folder so if they were
installed in the program files directory of you old XP install then those
items would/should still be in there.
Best bet is to check whats in there and see if theres anything you want to
save before doing a blind delete of the whole windows.old folder