Made the switch to Avira Antivir Free on my new set up. I'm very pleased so far. I like Avast too but compared to Avira it seems to be a little more of a resource hog. Functionality I don't know how they compare...I assumed both do essentially the same thing.
I highly recommend using Malwarebytes, run it about once a week is a good policy. It is however not an anti-virus program but nonetheless good to mention.
Okay, so I'll probably take heat, and I used to be on the "free" side too, but...
My current recommendation is Norton Internet Security 2010. If you are savvy enough on the web and can configure your router & firewall, then you can save some $$ with Norton Antivirus 2010. It is no longer bloated (They really cleaned things up since 2006), and does a very nice job. It also performs full scans quite rapidly.
I know people swear by AVAST (I prefer AVG), but the 4 computers of my in-laws have been infected multiple times EACH and they run AVAST. Yes, it's because they are downloading lots of files (some of which are NOT legit). Norton can actually step in and block those downloads (depending on my preferences.).
Download & Install the following 3 freewares. It forms a free complete security suite for your system!
1. Avira Anvivir - http://www.free-av.com/en/download/index.html .
Avira is best & light solution for excellent overall system protection & internet security with real time updates, .