Determining what service is tieing up computer

How can I determine what service is tieing up my computer?

I am running Vista Home Premium, x64 w/Quad CPU. At various times, the
computer will begin disk operations which completely tie up the computer;
CPU use is minimal but response time drops to 30-45 seconds at best. There
are no user applications running.

The only things I can think of is the indexing service or, perhaps, a
background defrag operation. But I haven't been able to pin down the
service. Any help?

Ray Titus