I use the free version of Avast (V7), updated from V6 no problems. So far I'm impressed, especially by the cloud streaming updates. I like to keep things lightweight, so do not use the WebRep or Autosandbox. In conjunction with my other security, and of course Shane's tweaks
, my system is snappy and safe.
I use Avast 7 (free), MBAM (free) on demand, MVPS Hosts file, Sandboxie, SpywareBlaster and Windows firewall (also behind my Netgear router firewall). My browser is Firefox 3.6.27 with Adblock plus (EasyList & Malware Domains), and Norton DNS....It may look like a lot, but uses very little RAM and all "play nicely" together with no conflicts.
I hope that offers some help.