I've also noticed that the cpu fan speeding is somehow related to the Intel(R) Desktop Boards FSC Application Service. As soon as I stop this service the fan returns to it's normal speed.
If I stress the cpu with prime95 afterwards, and monitor the fan speed with Speedfan, it behaves normally.
Is it possible that this service behaves oddly on XP SP3 ? I've noticed that after installing the SP3, Realtemp core temperature readings were a little strange, most of the time the cores would have the same temp, even though it wasn't true. On Service Pack 2 Realtemp behaved normally.