Investigating the hard-disk is most probably a waste of time. Some bad blocks on the disk don't manifest with spontaneous resets.
I would usually agree wholeheartedly with this statement, but... but.. I recently had some issues with my computer that traced back to some borked data on the hard drive, or a bad sector. Basically it would run along fine until I touched anything to do with Oracle 10g. Turns out that there was some bad data or a bad sector in that particular segemnt of the hard drive. Every time the OS needed to do something with Oracle, it would reboot the computer. If I tried to delete a file in that particular directory, reboot... start up Oracle.. reboot. It was only corrected after I did a Reiser fsck That said, I have to agree with the advice here... memory is almost always the culprit. Even if it tests OK after several passes of Memtest... just try swapping it... C.