Lag Spikes

I've been having this issue for a good few or so months now. Basically, Windows would start lagging and get choppy. When I'd go into task manager to see how much CPU is being used up, one of the cores is at a constant of being used at 100%, so it's using about 50% of my cpu power even when I click out and quit programs and nothing is running. Sometimes when I quit explorer.exe or log out of windows and back in, the problem goes away, but now it's just at a constant and usually a restart fixes the problem. So does anyone know what exactly could be the problem? I think I ran a virus check not too long ago and found some viruses which were deleted, but the problem was still there.

I have a Core 2 Duo 2.1 ghz process and 2gig of ddr2 ram, 2 200gb hitachi hd's and I'm running Windows XP SP2...if that info helps.

Thanks in advance!

 

Thanks! I will read up on that and report back if I still have any issues.