I'm totally guessing here, but I remember it seemed to be around the time I started playing Grand Theft Auto III, and had to install the WinXP patch that was apparently part of service pack 1. Also, around the same time period I had a hardware problem and WinXP suddenly asked me to re-activiate, which I did without any problem.
I just throw these out...they may have no connection, but it's worth a shot to fix this rather irritating problem.
P.S. Where can I get the manual updates/patches?