Microsoft Security Essentials
As you may or may not be aware, Microsoft has decided to release their own (Free) Anti-virus application. It's called "Microsoft Security Essentials" and it's currently being beta tested. I have used it and it seems to be a competent entry into the anti-virus market.
Microsoft has recently closed the beta test and they will not allow you to download the application directly from Microsoft. However, if you download the program from somewhere else and install it, the software will function as described. Auto updates work and the database updates download just like they are supposed to.
If you are interested in trying the program but have been unable to get it from Microsoft you can still download it from Softpedia (and probably some other sites as well.)
Click here to visit the Softpedia product information page (Where you can download the program).
Click here to visit Microsoft's Official Website for MSE.
UPDATE: Microsoft has now release the final version of MSE and it can be downloaded from the official website at the link just above.