For Free anti viruses, at least according to a report by av-test.org, Avast is significantly better than AVG:
http://antivirus.about.com/od/antivirussoftwarereviews/a/freeav.htm
They have failed to test one major new anti-virus, AOLs ActiveShield (I haven't tested it):
http://www.activevirusshield.com/
This uses Kaspersky, which is about the most effective scanner in existence, but not so light on resources.
For pay anti-virus:
Avoid Norton and McAffee; both are pretty shoddy products with monolithic support. After doing an audit where I work, a large number of different complaints led back to Norton which was used site-wide. We have licences now for FSecure and Sophos. FSecure is more solid than Norton (better detection rates and far less problems), but uses huge amounts of RAM and CPU. Sophos is more resource friendly, BUT for a virtual machine, I have to say there is only one ideal contender:
NOD32
It has steller retroactive scanning, is less than 1% behind Kaspersky for breadth, and it uses less resources than
any other AV I've tested. On a VM, keeping resources down is really important, and NOD32 has no competition. It is the only AV currently with solid rootkit detection too. Indeed, according to the very useful anti-virus independent testing site, av-comparatives.org, NOD32 is the best AV product of 2006 (the PDF summary is a very informative document):
http://www.av-comparatives.org/seiten/home.html
http://ssl1.at/av-comparatives.org/seiten/ergebnisse/summary2006.pdf
