Amazon.com has the excellent Kaspersky AV for free after a $50 rebate (shipping is free, too):
http://www.amazon.com/Kaspersky-Lab-KAV60B...2268&sr=8-2
For US customers. using this link, you can upgrade to version 7:
http://usa.kaspersky.com/store/renewal.php
Remember that once you purchase this, you can also use it for competitive upgrades when other products have rebates offered.
Even if you currently have a payware AV program installed, you can wait until its license expires and then install this one.
I'm not 100% sure, but I think that this program's license doesn't start until you install the program. Perhaps someone else knows if the license starts from purchase date or installation date.