Mike, supposedly BitDefender 2013 Antivirus has worked out the kinks that plagued 2012. Though I suspect part of your problem is that you left the automatic updates on. If you do that, it will constantly look for updates, thereby tying up your system. If you uncheck it and do manual updates you might be surprised at how light a footprint it has. Though I haven't updated to 2012 from 2011 yet.
I've pretty much tried most of the major ones. AVG was nice and light as was Avira, but both missed several nasties. And unfortunately AVG is so popular that some turkeys consistently write anti AVG trojans that disable it. Avast! Home Edition is nice, but only if you don't have a lot of music files, particularly FLAC files. It really gets bogged down scanning them. Eset is pretty light and seems to catch the majority of the nasties. That may be what I used in the future, especially since there are specials on it that make it very reasonable and it doesn't block my g/f's websites that she uses.
I currently used Bit Defender 2011 for anti-virus on two boxes and Avast! on another. Bit Defender seems to have one of the best detection rates that I've seen.
As far as anti-malware/spyware, I use the PRO version of Malwarebytes and Comodo Cleaning Essentials. I got a lifetime subscription for $15 on Valentines Day for Malwarebytes. It's nice in that it has an active web blocker for malware. Not that BitDefender doesn't usually catch things, but every once in a while I will see a Warning! from Malwarebytes. Comodo Cleaning Essentials(CCE) is a nifty free tool from Comodo that has caught many a nasty. It's a nice backstop. And it's helped me de-nastify friends and my g/f's computer many o' time.