In my experience with XP Pro (which was as short as it was frustrating) service pack 1 was not even available. I bought XP Pro SP2, and still had a bunch of upgrades to download. The only way I could stabilize the OS was to visit MS's safety.live.com site so they could dig into my computer (remotely) and fix bugs.
Maybe start there?
By the way. safety.live.com is a service by Microsoft that when you click "safety scanner" and then do the "full service scan" does sniff into your computer. It takes forever, and may not be for the "private minded" type of person, but is worth it if you are having OS issues.