Is my problem Win XP

[Whitelightning]

I am having a small problem and I think it is with either XP pro or an update from microsoft. I am using speed fan 4.40 and when I shut my folding down it shows that something is using almost 50% of one core. I have looked at task manager and nothing is running and shows system idle at 99% yet in the performance tab it is showing the cpu usage as 40-50%. I have formatted and re installed xp and it seemed to have fixed the problem until i did my updates and installed SP3....I need both cores for folding. Does anyone have any idea what could be causing 1 core to be utilized when nothing is running? Could it be SP3?