hard drive spinning fast for no apparent reason

My hard drive has recently begun spinning for apparently no reason. I can reboot and disable extra items in Win XP (via msconfig) and after reboot the system idles fine for awhile. But in less than an hour, it starts working: the hard drive starts spinning very fast and it will keep spinning fast for hours, if I let it.

Virus scans (w/NOD 32) are fine. Spyware scans Adware, Ada-ware, S&D Spybot, F-Secure BackLight, Registry Mechanic, Win Defender, Malicious Software Removal Tool, all come up negative.

I need to find out how to isolate what’s using up my system resources. Googling has not turned up much.

Any help, or referral to websites for help, would be most appreciated.