Win2k3 server freezes randomly.

I have Win2k3 OS server. PowerEdge SC420. Past few months we have been having these freezes, which need a hardkill and reboot.
I have done the following
a. CheckDisk on all the drives.
b. Defragged all the drives.
c. Ran the Anti Virus
d. Increased the memory by 2 GB
e. Has two built in fans for 1. Power Supply 2. Motherboard Heat sink. 3. A Cooling fan.

Do you folks any idea why the server is freezing.

This is the event viewer log (system)
"The previous system shutdown at 1:46:50 AM on 7/22/2008 was unexpected." This was logged at 7: 56 when one of the staff switched on the computer.
The entry prior to it is at 10.36 PM "The Microsoft Software Shadow Copy Provider service entered the stopped state."

This is the event viewer log (Application)
At 10.25PM "End Verify of 'D:' 'The operation was successfully completed.' Consult the backup report for more detail. " This was a backup operation of D Drive.