John,
Relatively speaking, an anti-virus program is a simple matter of user choice.
Amongst the freeware, AVG is one of the most popular due to simplicity of installation and the updating feature.
Avast, however, is rapidly taking the lead in both freeware and payware (the Pro version). Avast is constantly in development. Its' auto update feature for BOTH database and program probably now top the list of great features!
I would be wary of any vendor proclaiming 100% detection in all cases at all times, but you need to look at how viri are detected, and what detection system is used.
Avast does an outstanding job of detection for known virus types, while keeping system resources reasonable (maybe not as low as some would like, but certainly well within acceptable limits)
To be fair, I use both AVG in one of my systems, Avast Home in another, and Pro in a third. I do this mainly because one system uses AOL for an ISP which cannot be protected by Avast On Access Mail scanner since it does not use POP3 or SMTP Protocols. I have yet to be "stung" by a virus with AVG in place.
As I said earlier, it is up to the user to dertermine what they expect and how they use their system. I like AVG, have used Norton (bah!), Innoculate, and others, but I LOVE AVAST!!!!!!
Enough said. Good luck
techie