Scheduled task/reboot

Has anyone heard of scheduled tasks causing random reboots or shutdowns?

I've been having problems at work with seemingly random reboots on a w2k machine. One year old Dell. At this point it hasn't happened more than about once every two days or so. I've got no clue so far as to what's causing it. (I haven't had time to figure it out. And it's so seldom, it's hard to test for)

However, I've set this machine to download NAV updates from an FTP site everyday at 4:31 thru task scheduler. I was watching the seconds tick down to 4:31 (rare idle time for me) and as soon as 4:31 hit, the machine shut down. Not a reboot this time.

So I did some checking and found NAV LiveUpdate, (unknown to me) was set to check every 5 minutes. I've now shut this off.

So, any comments?

My first thoughts were either heat or a corrupt driver, possibly USB. Event log is showing nothing unusual.

Edit: I set the machine to not reboot on an error so I would get a blue screen error message but it reboots anyway.