Just a couple of suggestions, If you open the case, does it still happen? Most reboots I have seen are heat related, check all your fans (especially your CPU fan). Goto http://www.memtest86.com and download an ISO. Burn it to CD and boot from it. This will test your RAM as well as your CPU (since the CPU controlls the RAM). See if you get any errors, run the test 3 times if you get errors, and they are at the same place each time, then you have a bad stick of RAM. If there are errors but they are not at the same place (same megabyte range on each f the 3 tests) then you have a heat issue, or an issue with your Mainboard or CPU. Last but not least, run a chkdsk that scans the entire drive for errors, or goto http://www.grc.com and purchase SpinRite (I think it is around $60). This tool will scan your drives for errors and recover any data from bad sectors. It alsy will maintain your drive by making sure all the 1's and 0's are read correctly. to run a chkdsk type this at a command line, chkdsk /f /r c: or boot from an XP CD, hit r for the recovery console and type this chkdsk /p /r c: Post back if you have any more info.