Microsoft's Security Essentials has left beta status and is now available as a "real" product. I haven't personally tried this product yet, but hey, at least the price is right. Keep in mind there are other free antivirus products out there like AVG, but typically they are only free for non-commercial use. Apparently, Microsoft is not making a distinction in how or where you use their product. If you want to give it a spin, the link is here.
To Your Success,
Tim