Personally I and many other people wouldn't suggest AVG free, it tends to cause problems with the system sometimes. Same for Norton and Mcafee. I think any antivirus company which offers a "system optimizer" with a registry cleaner shouldn't be trusted, that's only me though.
@grammydragon: Webroot SecureAnywhere™ AntiVirus is not bad from what I've heard. Here's what Quietman7 (Microsoft MVP since 2007 and is a moderator on many security forums) has to say on the subject, and I fully agree:
There is no universal "one size fits all" solution that works for everyone and there is no single best anti-virus. Every vendor's virus lab and program scanning engine is different. Each has its own strengths and weaknesses, and they often use a mix of technologies to detect and remove malware. You may need to experiment and find the one most suitable for your needs. For more specific information to consider, please read Choosing Your Anti-virus Software.
Free antiviruses detect just as well as paid ones, just depends if you want extra features. ESET NOD32 and Kasparsky are often two of the highest rated paid antivirus programs.