I once spend ages on NT4 trying to trace this. There is a freebie program called FILEMON which shows you which bits of the HD are being accessed. Windoze writes to the registry every second, roughly, and also accesses something on every hard drive that's installed even if it is used only for data not related to windoze itself.
There is a lot of stuff on the web about how to reduce this HD activity. One starts off disabling various network printer broadcast etc processes, and one can get rid of most of it, but not all.
I think XP must do something different on laptops otherwise their HD would never be able to power down.