Microsoft Security Essential(MSE) has just shed its beta label yesterday Sept 29, 2009 and early reviews have been positive according to MSNBC (lol affiliate of Microsoft).
Anyways, what matters is AV-Test GmbH, a trusted independent source has given MSE's Beta version a good early review stating, "All (viruses) were properly detected and treated by the product. That's good, as several other (anti-virus) scanners are still not able to detect and kill all of these critters yet."
If you don't have a good free legit anitivirus, then you should check MSE out. I am trying it and it looks good so far on my labtop.
It's based on ForeFront, which I have been using on my server and it's totally kickass. Low memory footprint and does find a lot of things that Symantec seems to miss.