From the Windows 8 Event log.
Code integrity determined that the page hashes of an image file are not valid. The file could be improperly signed without page hashes or corrupt due to unauthorized modification. The invalid hashes could indicate a potential device error. (I did a disk scan and it came up clean)
\Device\Harddisk\Volume2\Program Files\Bitdefender\Bitdefender Windows 8 Security\active virus control\AVC3_00172_002\avcuf64.dll
avcuf64.dll Digital Signatures
Name of signer: Bitdefender SRL
Digest algorithm: sha 1
Timestamp: Nov 9, 2012 6:13 PM
File version: 3.10.7143.4226
Size: 535 KB
Error freezes system and requires a manual shutdown and cold restart.