Reading your question in detail it seems you have the wrong end of the stick. You DO buy the software, it is the data updates that either come on annual subscription (commercial AV) or is free.
If you don't get the updates your AV will continue to work, but will not know about, or be able to spot, the patterns that indicate a recently created virus.
AVG do a paid for one, but they also do AVG free, but you may need to hunt their site for it, or take a short cut using Google. It works as well as the others. IMO they are all much of a muchness, with one being slightly better one moment, another next time.