I and several of my friends have had this problem in the past.
I am still unaware of what causes this problem as it can be directed to so many different pieces of hardware, software and configurations, but I eventually resolved the problem.
To begin with, I have the same motherboard as you (A7N8X-X). The first thing I recommend to do is flash your bios. Goto the Asus website and get the latest software for your motherboard. Theres a program on the disc you recieved with the motherboard that can automatically find and install the latest software, therefore this will simplify the process.
If problems persist, go into bios and check your settings. See what your cpu is clocked at, and if it's overclocked to some extent, default it to it's original setting. This is to ensure there is no overheating taking place, that is causing your computer to reboot. It is unlikely though, because you will be informed of this if you have installed the software which came with the motherboard (asus probe). But then again, temperatures displayed on screen are not always correct.
The final thing I can recommend is formatting your computer. I formatted several times (I was using XP Professional at the time also), and although I recall problems persisting, this could have a positive effect.
Well thats all I can think of at the moment.