CPU usage is always 100%

My problem is that my CPU usage is always 100%. It never even downs to 99% It's always 100%. At 1st I thought it may be a virus. I format my whole hard disk with new partition, installed windows XP several time, but the CPU usage is always 100%, always. I can see it in task manager. In Process tab, I see that the process "System ", it usually takes 99/98% of the CPU. And the whole CPU usage is 100% all the time. For this my pc has become slow. I even opened the casing, opened the CPU fan, cleaned it, took the Processor from motherboard and put that back again. But still it always shows 100%. What should I do to solve my problem ? I am facing this for about 2 months.