I have noticed the same thing. I have disabled the anti virus, and stopped indexing and it still continues. I got process monitor from sys internals and had a look.
Windows media player network sharing service was accessing the disk so I stopped that. Using Norton that came with the computer. On an XP I have Mcafee, and the disk was not this active.
But 2 processes continue to use the disk repeatedly:
explorer.exe is using a slideshow.ini, but I have no slide show setup !! how to stop this??
And the peer network services is using the disk, why? there is no other computer on the network at this time??
Did not notice this stuff going on with XP, its very annoying, when the computer is idle the disk should be idle!! Over time this adds up to a lot of disk usage, reduce disk life??