ive got the same problem. it all happened a loong loong time ago... (phases to next scene)
i thought 'hmm, why not upgrade from XP pro to XP media centre?' so that i did, it just so happened to be not so legitimate, and appeared as if by magic on my harddrive... during the installation all was well until it loaded windows for the first time, where i got a load of missing file errors, lost parts of my admin control and firewall control.
i could not access ATI catalyst, or uninstall certain programs... i was screwed over but couldnt be bothered to reformat or repair to XP pro again.
basically... have you had a major software upgrade recently? if so, thats your problem.
(I DO NOT CONDONE DOWNLOADING ILLEGAL COPIES OF THINGS, THIS STORY IS FOR FICTIONAL REFERENCE ONLY AND ALTHOUGH ENTIRELY TRUE, STILL FICTIONAL, SO DONT PUNISH ME)