For a good anti virus scanner look for the following:
->Scanning accuracy
->Ability to clean threats along with detect them
Virus Bulletin is a site that compiles past and present results of a scanner's performance in its ability to detect all known 'In the Wild' threats at the time of testing. Below is compiled total of 52 sessions, including the most recent of June 2007
Mcafee - 32 pass, 19 fail, 2 no entry
Avast - 24 pass, 19 fail, 10 no entry
AVG - 18 pass, 22 fail, 31 no entry
Avira - 10 pass, 1 fail, 42 no entry
I would not go on these test results alone but use them as historic baseline. Consider PC requirements whereas some scanners may use more memory and resources to scan. There various reviews from CNET or Firewallguide.com for other things to consider. All of these scanners listed above are certified by either ICSA Labs or Westcoast labs in their ability to not only detect threats but to clean threats as well.