Always on reboot, and always before the POST. It's the same symptoms every time, and it always happens after the machine has been on for several hours (we keep it on all day, and shut down at night). I've had it occur in three different scenarios: 1) I do a warm restart after a software install, 2) the system will freeze completely, and I'll have to shut off power and then power up again, 3) the system will shutdown and restart randomly and without warning; most of the time this happens while surfing.
It happens once or twice a week. Not enough to drive me bananas, but enough to cause me concern that this may be the beginning of something more sinister.
Sygate and AVG always running. AdAware scan three times a week.
I'm leaning towards the overheating theory too, because if I leave the machine off for 5 minutes the problem goes away. 50C seems a little warm, but not drastically out of whack.