Random reboot in Vista 64

Hi
I had a strange problem occur this morning, and I was wondering if anyone can shed some light on it.
I run a dual boot system with Vista64 and Xp home, they are installed on separate drives, Vista being the OS I use most of the time. I booted into Vista and started doing some eBay listings and suddenly the system tried to reboot, listing a possible hardware error. I then tried to boot into XP and the same problem occurred. I managed to boot into safe mode (Vista), and tried a system restore, and again the system closed down, but did not reboot. I switched off the power, and left everything for a couple of minutes, then rebooted into Vista ok. The point that is concerning me is that this random problem affected both OS’s, I realise that System Restore has possibly done its job, but has anyone any idea what the problem may have been. All temps. Etc. are fine. I know it could be a thousand different things, but hopefully someone has had a similar situation occur and can shed some light for me.
Thanks in advance

NB I have since booted into XP as well with no problems.