Task Manager may be able to show you which process is doing it by observing if you have a process with high CPU usage(in the CPU column).
If need be, sysinternals.com has excellent programs to help pin it down. ProcessExplorer.exe is one.
You didn't say if this happens every day. My virus scanner wants to do a complete scan once a week. It wants to scan both my big hard drives. I usually kill the scan after a half hour or so, sometimes sooner.
I also disable XP's "Indexing Service". I guess this will cut down on occasional high disk usage also.