Looking over the event log shows the last message before the reboot is:
2/10/2006 10:04:33 AM EventLog Error 6004
A driver packet received from the I/O subsystem was invalid. The data is the packet.

This error is not usually critical but is usually indicative at some sort of network driver/filter problem. This could be the actuall driver, or something hooking the NDIS, like a rootkit or even just your anti-virus. Now lets look at the next interesting event after the reboot:

2/10/2006 10:18:00 AM Save Dump 1001
The computer has rebooted from a bugcheck. The bugcheck was: 0x0000008e (0xc0000005, 0x80839148, 0xf7546c60, 0x00000000)

A bugcheck status of 0x0000008e or 8e, is a KERNEL_MODE_EXCEPTION_NOT_HANDLED error. The information in parenthesis is the exact address of the code that cause the unhandled exception. Since the last event before the exception was the 6004, it looks like the network driver/or filter is taking big ****. I'm still nit sure whether this caused by hardware or software (aka re-infection, AV, etc) You need to eliminate the possibilities. I would try this:
First, boot to a memtest CD and test the memory. Then move the NIC card in the server to a different slot. Make sure you have the newest drivers from Intel for the card, and let the machine run. If if it still crashes, move it to an isolated network and remove AV software and anything else which may be hooking the network drivers. Give it some kind of network activity and let it run. Post back with anything you may turn up.

-Maestr0