Random reboots under Windows x64, HELP.

Hi,

I own an Athlon x2 processor (4400+) and I'm currently running Windows
XP Pro sp1 x64.

My system specs are as follows:

Asus A8n32-SLI Deluxe MB
Asus EN7800GTX Nvidia 256 Mb video card
4 x 512 mb PC4000 OCZ Gold Rev. 2 Ram
250Gb WD 16 Mb SATA2 HDD
500 watt Antec Fanless power supply
creative x-Fi sound blaster Xtreme Music Sound card

plus all the usual peripherals etc..

I have a problem with my ssytem that is driving me insane and I wonder
if anyone here can help me.

My problem is as follows: My system randomly reboots itself with out
any warning. The reboot are sometimes completely random, as in I'm not
using any software and the computer is just idle and other times tne
reboots occur when I using anything from MS Office to playing a game.
I've scanned my system for viruses and found nothing, I've useed a
bunch of anti spyware programs and deleted anything those programs
found, but still the reboots occur. What's going on???? My ssytem is
relatively new and this is driving me crazy. I've intalled all the 64
bit compatible drivers I can find for my hardware and ther doesnt seem
to be a problem so what gives. Sometimes i get an error in windows
after the reboot that a serious problem occured and that it was
potentialy hardware related. Could it be, if anyone has a clue or
wants to help me out please go ahead.