Recently, to be more extact, 23th June 2009, Microsoft released the beta of Security Essentials which was the previously Windows Live One Care.
Allow me to quote this from wikipedia,
[quote]Microsoft Security Essentials (codenamed Morro) is an upcoming gratis antivirus software created by Microsoft that will provide protection from viruses, spyware, rootkits, and trojans for Windows XP, Windows Vista, and Windows 7.
Amy Barzdukas, senior director of product management in the online services division at Microsoft, said: “This new, no-cost offering will give us the ability to protect an even greater number of consumers, especially in markets where the growth of new PC purchases is outpaced only by the growth of malware.” 
I will say that the installation is very smooth and the virus scanning relaly run quietly at the background. These are some of the screenshot which I have taken while installing.