I guess one chooses by browsing many user (as opposed to sponsored) reviews/reports, and then trying a few out. Any reputable AV will have a full-featured trial version. Some have free versions. (Including AVG.)
I've read excellent user reports about Norton 2009. Earlier Norton products were bloated and took over the machine. Apparently the 2009 version (both AV and NIS) is light, fast, and effective. It has some converts.
I use Avast! Home (free). The only things disabled in the free version compared to the paid version is the ability to schedule scans, and script blocking. I don't need those features so it doesn't bother me. Some other free versions disable or partly cripple detection/cleaning ability. Avast has never given me a reason to want to change. It just works.
Avira has a happy following. Always scores well at test sites. (as does Avast.)
McA has a Norton-like reputation for bloat. So do some versions of KAV. Comodo is a comprehensive package, basically a suite for free. The firewall/HIPS components are among the best. The AV component is still young, and will take a little longer for detection rates to be "up there" with the best. The HIPS is good enough that maybe that doesn't matter. The components can be installed separately.
With Avast, I'm also using a two way software firewall, Threatfire, (a behaviour blocker) and the MVPS hosts file/SpywareBlaster for blocking known bad sites, and Comodo Memory Firewall for buffer overruns. All seems to work nicely.
That's an example of making your own free security suite.