Hard Drive Activity

I have a Toshiba Satellite 2400-S201 laptop with Windows XP as its Operating System. I have noticed that, while the system is in idle, the hard drive likes to kick in every now and then. It makes a sound as if it was reseating on a track or something like that. It also tends to, every now and then kick in with a lot of activity. When it does this, I notice some processes that take up some paging, disk queue, and processor time resources. These processes are: svchost.exe, helpsvc.exe, crss.exe, dfrgntfs.exe, and defrag.exe. I know what defrag.exe and dfrgntfs.exe are for which I guess it is normal on Windows XP. Can anybody tell me what the other programs are and if it is normal for a hard drive on Windows XP not to stay “quiet” when the computer is idling?