Perhaps you have been "adjusting" the prefetch folder, or other
optimization? And the system is straightening thing back up?

Windows XP does not enter standby after the exact period that is
configured in the Power Options profile
http://support.microsoft.com/kb/899975/en-us

mbs6 wrote:

> I have a machine running Win XP Home and whenever the machine is idle for a
> while the system disk goes nuts with reads and writes and never stops. I
> finally determined that defrag.exe and dfrgntfs.exe are the processes
> responsible. Somehow they are starting up whenever the machine is idle. It
> is driving me crazy as it never completes and causes my disk drive to always
> be in use. I suppose somewhere in the bowels of windows it has decided to
> take idle moments to optimize data on the disk. I DON'T WANT IT TO DO THIS
> WITHOUT ME ASKING IT TO!
>
> Somebody, somewhere at Microsoft knows why/how this is happening. Please
> clue in the rest of us.