Once last week mt PC rebooted, I wasn't home. Then, while I was sleeping this morning it rebooted again.

I have no clue what's causing it. I just built my PC a little over 2 months ago, and it runs perfectly, even when gaming, and it's never crashed on me while using it.
Now, my question is how can I figure out what's causing this? There's no Windows has just recovered from a major problem error or whatever it says, so I have no clue what the deal is.
specs:
XP SP1
AMD 3200+
Gigabyte GA7N400 PRO2
4 Western Digital 80GB SE's
9800 PRO
Kingston HyperX PC3200(2x512)
Turtle Beach Santa Cruz
CDR & DVD
Vantec 470 watt PSU