chances are that either the xp install or the PS7 install went wrong. What I suggest is to uninstall PS7 (copy your presets folder and save it somewhere safe if you have custom brushes and actions first then put it back afterwards).
Run sfc/ scannow (go to start>run and type that in, this will check the fundamental windows files) you may/will need your windows disk. If you don't have one i'll tell you how to do it the long way.
Then reinstall PS7.
Also use regscrubxp (search google for it - it's freeware and it's safe). It comes recommended by a pc journalist i know as being one of the better reg cleaners for xp. That and easycleaner from toniarts which is also freeware.
give that a go and see how you get on,