As an IT guy in a good-sized law firm, I’m sometimes asked to make recommendations for anti-virus software.

For real-time protection that is always running on your computer, I like Microsoft Security Essentials.

Microsoft Security Essentials provides real-time protection for your home PC that helps guard against viruses, spyware, and other malicious software.
http://www.microsoft.com/security_essentials/default.aspx

If you don’t trust Microsoft, SUPERAntispyware has a terrible name but a good track record.

The SUPERAntiSpyware FREE Edition offers real-time blocking of threats.
http://www.superantispyware.com/

The SUPERAntiSpyware Portable Scanner can be run from a USB flash drive or CD without installlation.

The SUPERAntiSpyware Portable Scanner does not install anything on your Start Menu or Program Files and does not need to be uninstalled. Just download it and run it whenever you want.
http://www.superantispyware.com/portablescanner.html

Personally, I run Microsoft Security Essentials, and then do supplemental scans with the SUPERAntiSpyware Portable Scanner.