Superfetch is the likely culprit here I think, One trick I have found even in high end systems, is to wait until disk activity stops after boot up (even on my top end self-build it's coffee time) this seems to give the system time to settlt, then all is well. I have it on some authority that SP1 will address this issue.
Not seeking to create an arguement FE but I did notice when I was trying Vista that the hard drive did seem to work away quite a bit even when not used. I thought, perhaps wrongly that it was windows itself doing its housework such as defragging etc. Was I wrong? I thought that at times the system was idle windows itself did stuff. It didn't really bother me as my 2 drives are reasonably quiet and are down next to the floor out of immediate earshot.
I have noticed this on older computers as i have two one 5 years old the other 3 years old this doesn't seem to happen on my new computer that came with Vista on one of my older computers i changed the graphic card to one that will work with Vista it was a Radeon X1550 AGP interface.