asus p5b rebooting

its been about a month now
and every often my computer just reboots itself on its own
i have no idea why its doing that

my cpu temp are around 35c, and my harddrives are running at ~40's

i have the antec 430wat psu, and i think i have plenty of juice, so i dont think its the psu

does anyone have an idea how i can solve this problem?

from what i researched people are telling me to re install video card driver, and others are telling me that I have to go to bios setting and change something? (whats that, im really noob)

oh yeah and my computer is not over clocked.

any help or suggestion is appreciated

thanks