The case comes with a 300w antec PSU. Antec's are usualy pretty solid, so I don't really think that's the problem. Did you change anything else besides upgrading the proccessor? Are you using the stock cooling that came with the CPU? Prescotts generaly run warmer than northwoods, so the small case it's in may not have sufficiant cooling. Are you using any software to monitor your temperature? If you are using windows XP did you disable the automatic restart if it errors?(right click on my computer, click properties, go to the advanced tab, click on settings for startup and recovery, and uncheck the automaticly reboot). Are you running any programs when it reboots, or does it happen when it's idle? Have you checked for viruses and/or spyware?