For your windows 7, you'd have to reinstall windows xp if you dont wanna stay with it, as simple as that.
Btw though, you can make your trial longer for windows 7 when it reaches its end.
When it's about to end, open the command line interface and type in "slmgr -rearm" (without the quotes (reboot after that)), it should add 30 more days to the trial, you can do this up to 3 times I think. Anyway, if it fails and you can't do it anymore, it will tell you that you reached the limit number of times you could do it.
Edit - I think it has to be run as admin too .