AVG is garbage now. It used to be great, but really it can't hold its own. Ever since AVG upgraded beyond 7.5 it has been becoming more and more like McAfee.
Check out
http://av-comparatives.org for actual performance tests to see how the products stack up to each other in all categories, even how much they slow down your system.
December 2009 AV Comparatives summary report
This is their newest report, and to my surprised Symantec has really cleaned up its act performing best overall and even having a very low resource footprint. Still, like most here I don't trust symantec and would say you should go with #2 or #3 Kaspersky or Eset NOD 32.
AVG still works and is decent protection for freeware, but it is not better than the other products listed in that report anymore. They had a good thing going until they decided to make extra features like the AVG Link scanner. It was at the point of AVG making version 8.0 that they seemed to switch R&D off of potent virus detection/removal and switch it to silly features like the link scanner. Also, it was at this point that AVG stopped being so conservative with system resources and began demanding more from PC's. Still, people having faster and faster computers has made that change hard to notice for some. I'm not just bashing AVG to be an ***. I have recommended and sold it to hundreds of people (Currently at my job at Officemax and formerly when working at a computer repair shop) and it upsets me to see how far from the top it has fallen today compared to a few years ago.
Also, if you actually have an infection it's very hard to beat
Combofix and
Malwarebytes. They're not proactive scanners so it's good to have a real Anti-Virus product for front line protection, but these two programs easily surpass Spybot at actual spyware/malware/virus removal. Combofix is a beast that doesn't afraid of anything.