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 want to install software that runs all the time, but just want to run a scan now and then, I’d suggest Windows Live OneCare safety scanner.
Windows Live OneCare safety scanner is a free service designed to help ensure the health of your PC.
http://onecare.live.com/site/en-us/default.htm
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.
i recommend Malwarebytes’ Anti-Malware for spywares.