Disk thrashing

Hello,

My computer was completely corrupted and the disk was thrashing almost constantly to the point where I couldn't use the computer at all. I bought a brand new disk, (same kind), and installed a fresh copy of 10.4 then upgraded to 10.5 via upgrade disk.

My computer disk is once again thrashing. I'm very afraid the same thing is happening again and I can't find the source. It's frustrating and nerve wracking.

Things I've tried:
1. Stopping all unnecessary programs.
2. Looking at Activity Monitor for programs using lots of CPU time. None. It thrashes in the background.
3. Spotlight indexing was doing some of the thrashing, but I turned it off.
(As a side note, Spotlight would index disks which were in the Private tab)
4. Moved many files to an external drive. My system drive has 400GB (40%) available.
5. Turned Time Machine off.
6. Examined Console for error messages. The only messages are about iDisk not being able to sync, which is a separate help topic. I don't know if the issues are related. They may be if there is disk corruption.

Ideas are greatly appreciated. Thank you.