I suggest using Microsoft Security Essentials as a free security solution.  You must be running Windows 7 or Windows Vista.  Microsoft Security Essentials will not run on Windows XP.  This  is a good product, minor processing overhead, and unlike some other free products (e.g. AVG Free) it will NOT expire in a year.  So it provides a solid long term fairly hassle free solution.  http://www.micrsoft.com/security_essentials