Also another one which is free is avast! 4 Home Edition

But it is for home users and non-commercial use.

Some of the notable features are:
Antivirus kernel
Automatic updates
Simple User Interface
Virus Chest
Resident protection
System integration
P2P and IM Shields
Integrated Virus Cleaner
Network Shield
Support for 64-bit Windows
Web Shield
Internationalization

Download it here