How about the freebie from AVG (Grisoft)?
In my experience, with antiviral protections, they have to be current. Updated as often as necessary to counter emerging threats.
Freebies often are not updated and currently updated...they issue definitions and those remain until "next" updated, whenever someone gets around to it or whoever issues the freebie updates their definitions (many servers are maintained in this same way, which is why so many viral attachments and such are able to proliferate through email -- because various servers aren't currently maintained as to antiviral definitions).
So, you get what you pay for, literally, in this area. Paying people is a good thing when it provides incentive for a better-than-everyone-else standard, in my experience.