Why does my Windows XP machine reboot itself every morning?

My computer runs Windows XP SP3. It restarts itself every morning at 8am, judging by the Windows logs; all apps log a startup at 8am every day. Whenever I get to the machine in the morning (around 9am) I see that it's rebooted.

I've ensured that automatic updates are off, and that the power management settings are set to "always on." I'm not sure what else could be causing the machine to reboot, or even where to start looking.

Any ideas?

EDIT:
Just looking at the Event Viewer, I see a log message at 8:00:47am every day, which looks like a reboot log statement:

Microsoft (R) Windows (R) 5.01.2600 Service Pack 3 Multiprocessor Free.
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