Hard Disk runs continuously in Vista

I'm running Vista Business and from the time I boot each morning till I close down, my hard disk is running for most of the time. If I do some heavy processing it stops for a while. If I disconnect from the internet (broadband) after a minute or so the HDD stops. If I reconnect after a few minutes it starts up again.

With no programs running and having quit all peer-to-peer programs and with internet connected, checking in Task Manager, I can see the only thing using CPU time is System (NT Kernel & System 116K) and this often is using up to 25% of CPU time. With internet disconnected this drops to 0% although even when connected there is almost no internet traffic.

My virus/spyware checker (Trend PC-cillin) reports nothing and to me it looks as though it is something genuine in Windows Vista that is running.

Does anyone know of anything in Vista that could cause this and if there is any setting I can change to stop it?

Doug