Most common cause for spontaneous reboots, especially when the computer is doing some sort of strenuous task like playing a high end game, is the power supply being inadequate.
You'll probably want to get a power supply with an extra 50-100W capacity over what you've got now if you go with a good brand product like Antec. If you go with some no-name cheap unit, I'd go with a 500W unit to be sure you've got plenty left over since it's pretty much a given you'll never get anything close to the full 500W out of it.
Running Win XP Home with SP-2, 736 Mb Ram/ 1.82 ghz Pent 4, I will be running a game and the computer reboots. ARGGH. Game requires 512 mb ram 1.4 ghz Pent 4 with 128 mb video card. Which mine has.
I have turned off automatic restart so i can see the blue screen. I do have the notify Microsoft on. Of course I my device manager shows no problems. Any Idears?