If you haven't done this already, I'd run
Spybot S&D and
Malwarebytes to see if you've got anything. I'd also inquire as to WHAT you're using for anti-virus as well, since that might be compromised.
If you don't find anything significant, then I would also think that something in the OS has corrupted but I wouldn't be able to begin to tell you how it happened on the lack of details. Considering that you don't have your Windows XP copy up-to-date, I can't figure that it was a bad Windows Update install, for instance.
It sounds very much like a malicious program of some sort.