PC uses 100% of CPU

I recently upgraded an Asus P4B533 board to a 2.66Mhz P4. It runs XP. I download the newest updates at the MS site, and updated my Norton AV, and Spybot, and ran checks on everything. Worked fine…….I loaded up my new TurboTax (2003) and then downloaded the California patch to the PC.
I shut the machine down and left for a few hours.
Came back and tried to go on line, and the PC was very slow. TskMgr resource meter says system (not system idle) is using 100%, and my Norton AV is turned off, and won’t scan. Ran all the recent worm detections, and can’t find anything?
Any ideas?