You should never use 2 antiviruses on one system. Ignore most of what that guy said up there. Antivirus 2011 is a virus and buying it is feeding an operation based out of Europe designed to scam people. More than one antivirus program will actually compromise your system.
I've used Spyware Doctor for about six years now. I used Norton 2009 and Spyware Doctor at the same time, and Norton said there was a conflict, so I needed to take off any other Antivirus/Spyware software. I did, and Norton let a Trojan slip through. I reloaded spyware Doctor, and it cleaned the virus off after running Spyware Doctor twice. If a virus ever slips through, Spyware Doctor always cleans it off. I use AVG and Spyware Doctor with Antivirus 2010 right now. I purchased Spyware Doctor with Antivirus 2011, and was thinking about purchasing another to go with it, but after reviewing what AVG has found, I'll only use Spyware Doctor now, unless I have a problem it doesn't clear up. I know you aren't supposed to use two Antivirus at the same time, but I'm very leary about one catching everything. I've also thought about purchasing Kaspersky or Bit Defender, but everybody has their own opinion about each one, and it's time for me to use what I see is working.