Since I deal with college students and their computer issues at work, here is my run down:
Avast: Doesn't always get the virus you are trying to get rid of. Often slows down the computer with pointless start up scans, and needless addon options that are pointless.
AVG: Very lite, very easy to use, annoying when you miss updates. Often asks for unneeded restarts. Does a very good job, when it does do a full scan, which can take like a day if the computer is slow and old.
TBH: I have AVG on both my laptop and my desktop, but I never scan with it. Its like buying car insurance when you never get into an accident, its great if you have it, but if you don't have it, you're not going to lose sleep over it.
In the end: Have AVG for the little every day stuff, and if you think that you truly have something infecting your computer, usually XP Antivirus 2009(or 2010, anti-virus-1, or Antivirus 360), just keep MalwareBytes Anti-Malware installed and updated on your computer. It will remove everything like bleach removes color from your fine =(eGO)= gear.