Even though a lot of people here don’t like Windows Defender I have had no problem running it along with SAS and avast! in Real-time

On Demand scanners that I use include McAfee Stinger (Very useful for scanning Windows Backups Image Files in case you unknowingly backed up some Malware. It will also scan your computer’s Boot Sectors including the MBR).

I have also used MSFT Standalone Systems Sweeper. This runs from a bootable CD and/or USB Flash Drive and boots you directly to the program to scan.

Microsoft Standalone System Sweeper Beta is not a replacement for a full antivirus solution providing ongoing protection; [b]it is meant to be used in situations where you cannot start your PC due to a virus or other malware infection.[/b]

http://connect.microsoft.com/systemsweeper