I've had this before and forgot how the hell i fixed it (prob just did a XP repair install and upgraded the system all over again).
I hate this damn annoying error though, because i have an important email i need to read and i start to get the random problems!
Windows XP File Checker
Diskeeper 8 Defrag
Norton System Works 2003 1 button check+ Anitivirus check
Spyware check with AdAware (latest build).. just incase