Microsoft Security Essentials - Still The Best Free Option

"Microsoft's free Security Essentials package rates well at removing malware and running light, but what about dealing with brand-new, unidentified bad stuff? Turns out Security Essentials is still the best at figuring out what's bad-while also avoiding annoying false positives."

The AV Comparatives website have released a report detailing the results of how 20 of the main anti virus software products perform when their virus definitions are out of date.  The idea behind this is to test the heuristic abilities of the products to detect new viruses that have not been identified or fixed in virus definitions.  While the MSE product did not come first, it was the highest performing free product in the tests with a very creditable 5th place.