Thanks to all who have responded.
I disabled system restore and ran a scan of ALL files with PCchilin, also use SpyBot and both come up clean on a full scan... Still haven't discovered the source of the problem.
The winxp came installed on my computer purchased brand new a couple of years ago. I have accessed the registry in the past without problems and I've not installed any new programming since, so I'm pretty confused as to what's going on.
What next guys?