What causes this?

[Kevin J]

My computer all of a sudden out of the clear blue decides to reboot itself! No reason, just... Shutdown, reboot. Sometimes it does this 3 times in a row. Sometimes it doesn't do it for a week. Anyone know wtf would cause this? I'm thinking maybe a bad fan? I'd appreciate any help.

[BBill]

I just found this summary of the most common reasond for this
I usually suspect a bad power supply but there could be other reasons

Symptom: Your PC spontaneously reboots.

Solution: A long-standing mystery solved! If rebooting occurs in a PC that you've just built, try re-seating your CPU's heat sink. Make sure you're using the proper thermal gel and spread it evenly between the heat sink and the processor. If inadequate amounts of gel have been applied or low-quality gel has been used, the system will reboot as the CPU heats up---and builds in the uneven "pockets" created by the uneven gel. Also: check to see if you've removed the protective sticker on the bottom of the heat sink (don't laugh---it happens!). And by all means, make sure your motherboard supports the CPU you're installing. If these steps check out and you're still experiencing spontaneous reboots, your problem may be one of the following:

Overclocking: We do not recommend overclocking. Memory Timing: The fix? Go into your BIOS and set your memory on "Auto" or at a more conservative setting and see if the reboot problem goes away. Oudated BIOS: Make sure you have the latest BIOS for your board. You can determine if your CPU is supported by browsing the BIOS updates of the motherboard's manufacturer. If you're running a Pentium 4 Extreme Edition and notice that it's only supported with the latest BIOS updates, you may have located the problem!

Inadequate Power: If you've made significant component upgrades---with the exception of the power supply---your power supply may be overstressed or failing due to heat or age. Finally, if you've migrated your OS and other files from machine to machine to machine, it may be time for a clean install.

[jeffs]

have you called sue yet?

[BBill]

good question - or maybe its not a good question, I dunno, who cares?