Avast is good, they have a 3 star rating with AV Comparatives, but the free version lacks important components that's needed for email, shopping, banking, etc.
The Pro version can be had, a single user license for $19.99/year, a 3 user pack for $29.99/yearly. For this price, it's well worth it. You can go to their site and compare features.
http://www.avast.com/free-antivirus-download
The promo price may not show up on their site, but once you download & install the free (or trial) version, then you'll be offered the discount. In fact, I have it on Win 2K, and was offered these deals just yesterday, in addition to other times.
Also, were it not for yodap providing the link for a 3 user pack of ESET NOD32, I'd probably have bought the 3 user pack of Avast Pro myself.
Cat