UPDATE

Antivirus Definition Updates

Microsoft Security Essentials Definition Updates

Microsoft Security Essentials for Windows 
Bitdefender Virus Definitions
Trend Micro Pattern File for Windows

Avira Virus Definition File Update

Comodo Anti-Malware Database

K7 Offline Updater
Zillya! Antivirus Definition Updates

AVG Anti-Virus Update

AVG Free Edition here

IKARUS Security Software Virus Database

eScan Weekly Offline Updates