How much is the memory usage for system idle?
The high number for the system idle process indicates your reserve of the CPU not being used. This is normal. My 3.2 GHz P4 w/hyperthreading indicates a "99" with only the browser open.
If you're running XP and don't plan to use the restore points, try turning off system restore. You can disable "NTFS Disable Last Access Update" by editing your registry. Also, you might try turning off your Indexing service ...that's assuming you haven't already tried those things. There's alot of software that might cause frequent writing and reading of the hard drive ...like automatic defraggers, network scanners, registry trackers, anti-virus etc...(but you'd typically know that they were running).
IMO, if you can't isolate the activity with the hard drive, it might the beginning of an eventual HD failure.