I am having the same issue, it started last Thursday, Periodic crashes with error 0x000000c5. Ran the windows debugging tools and the results weren't helpful. It seemed like the crashes would occur when the machine was idle for a while and I came back to it. I removed Avast 5 and the machine hasn't crashed since. I am not sure if it is related to a windows update or another program conflict? Just for kicks I tested my Hard drive and memory, no problems found.
The event log entry for the last crash was
The computer has rebooted from a bugcheck. The bugcheck was: 0x000000c5 (0x00000004, 0x00000002, 0x00000000, 0x82d3404d).
After these crashes, I would uninstall avast in safe mode, then re-install. The computer would be fine until the next day, then the problem would occur again. Are there some updates to the AV engine that don't require a reboot, but change the way Avast works when the computer is restarted???