I just experienced something strange in Windows 7. I could hear how the harddrive was used at regular intervals of about one second. The same thing seemed to steal my CPU timeslice (from my own application) at about the same regular intervals. I opened up the task manager and started looking at the processes there, but could not find any offender. No single process had page faults of CPU usage that had the same regularity, but nonetheless I started killing off services, such as Windows Search, Windows Defender, and so on.
Eventually the disturbance went away by itself.
What I did see (I think) is that sometimes one process -- any one -- would spike page faults and/or CPU usage at about once every second. Which leads me to belive that the cause of this was a driver. Is there some kind of defragmenting driver (or at least some type of driver) that sometimes does hard drive reads/writes at intervals like this?