You must understand I am not one who is a techie; however, I am relatively proficient with computers, and I have found the very same thing occurring with my XP OS. All I could do, was check this and that, in a matter of "well, it's not this, not that, not this."
After doing this for quite awhile, I found something that I do hope will help you, for MY PROBLEM, was that I used two anti-virus software, and two malware software, and these were not getting along.
I was using Avast Free edition, and I had bought a two-year copy of BitDefender [under $50], and because I was so sick and tired of this !00% usage of the CPU, and my mouse barely being able to move because of the CPU max, I did two things:
1/ I downloaded BOTH anti-virus software, and I downloaded only one malware, the free edition, and kept the Sunbelt Counterspy, which I had paid for. BitDefender was trouble to uninstall, and so I hope you do not have this one! I downloaded MICROSOFT SECURITY ESSENTIALS, which is a really good and FREE anti-virus, anti-malware and firewall, and I kept Counterspy, I think is the best anti-malware software around. Right now as I write, my CPU is working at 11% with three pieces of software working [WordPerfect; Foxit Reader - the best PDF Reader around - and free; and Advanced System Care Pro], and this machine has not run this fast, since I bought it. Windows XP Home, only 2 Gigs of RAM, AMD Athlon -3700, running at 2.2 Gigahertz.
Now, I was having this problem off and on for months - sometimes I would just shut the whole machine down, and do something else, really uptight because I didn't feel I could find the cause, but last time, three weeks ago, I'd finally had it, and I did find the reason - no viruses, no malware, too much software for security, and they would get all maxed out working together, and so I did the BEST THING I had ever before done, to increase my system performance. At least every second day now, I defrag and optimize, and the machines's like lightning! If you've got lots of security software, try taking one out of memory, just leaving one anti-virus working, and see if this works. OTHERWISE, scan, scan, scan!