the message is from your monitor, when it bails, your graphics card must send signals that your monitor cannot handle
for the cause of the crash, go to control panel=>administrative tools=>event viewer and in all the three catagories, try to find red crosses around the time of the system crashes
does the PC actually reboot (i.e. goes through POST etc) or just it just flick off?
If all else fails, find a really kind mate, and borrow his graphics card for about a week and see if the problem still occours (H.V frequency out of range)