Is the Indexing Service running? If so, it may be busy indexing the files on the hard drive, a process which, I understand, can take many week or months to complete.
Whilst the Indexing Service can be useful for fast searches of files on the hard drive when it is completed, I never used it and disabled the service.
On a friend's computer (WinXP Pro SP2) a problem started few days ago - endless disk activity. Yesterday he did a new and clean install, but that didn't solve the problem. It's a fairly new PC - Core2 + 2GB RAM. Except for this everything seems to be working, no error messages or anything else. Any idea?