We have a Windows 2003 SP1 server that reboots randomly throughout the day. It could happen in the morning, afternoon or evening.
The past 3 reboots (over 2 days) have shown the following in the event viewer.
1. USER32 Event ID 1076 Rason Code 0x8000005
2. USER32 Event ID 1076 Reason Code 0xa000000
3. USER32 Event ID 1076 Stop Error 0x00000077 (reason code 0x805000f)
I can't work out why, but also we have only just taken the tick out of Auto reboot, so we dont know what appears on screen when it reboots, apart form it goes back in to windows with no issues until the next random reboot.
Set it in "My Computer" -> Properties.
Blue screen should appear and display bad driver name.
By displayed name you can update some device driver or replace bad hardware.