One of my coworkers just pointed this program out to me and I've since downloaded it for both my personal laptop and my work desktop. I had been using the free anti-virus program called AVG. Now I'm officially using Microsoft Security Essentials as my anti-virus program.
It integrates great with Windows 7. It has a clean interface, runs in the background so it doesn't bog down your processor when you're using the computer and it's free.
Check it out here: http://www.microsoft.com/security_essentials/default.aspx