I had this very strange behavior for a few days on one of my boxen - the machine would reboot about 1-2 times a day for no obvious reason. The machine sits behind a firewall that blocks all ports except 1 - IIRC 1250 or 1251, for a Neverwinter Nights game server. The server is currently not active. That port is forwarded directly to the machine in question.
No spyware (other than M$'s), no funky weird apps.
Event log shows the Intel Etherexpress 100B going offline, then the next event is a "System rebooted due to a bugcheck" with a register dump.
Ring any bells?
Win2k Server, SP3.
Oh, and this appears to not be happening any more - I left the machine off for 2-3 days, turned it back on, and it didn't happen again. That's why I suspect an exploit, as opposed to a bug.