I too have also had this problem and like stated above (cant remember who by) the first thing to look at is the PSU, my prob wasnt the PSU exactly but the ATX connector that goes into the motherboard. Some have got really crap quality tin pins that over time just seem to lose their contact and cause the system to restart abruptly. I could work for 3-4 hrs one minute before it restarted and on other times I might get to work for 5secs, other times it reset before windows even booted. Anyways I changed the ATX connector from my power supply and hey presto no spontaneous reboot since. Hope this is of some help to someone.