Every 5 minutes avast tells me there’s some win32:malware-gen infected svchost on C:/Windows/TEMP folder. Nothing is behaving in a weird manner, it’s just that malware blocked notification that keeps coming up.
I didn’t want to do the exact same things recommended to the guy above because I was afraid something could go wrong, as even though the problem is the same, everything else is not.
Whilst this is possibly unrelated to your problem, but XP SP3 has been out for over 18 months and closes a number of vulnerabilities and has some security improvements. Not only that come July 13th this year MS will no longer release security updates for those with XP SP2 or lower, the clock is ticking.
I would also suggest a visit to this site, which scans your system for out of date programs that have patches to close vulnerabilities, http://secunia.com/software_inspector/.