After what i've heard Avast antivirus is among the best antivirus progs and avg among that progs you should stay away from. In a test avg did allow 59 worms to get through, but avast was one of the best in this test.
The program list below shows which ones stoped every worm.
Avast!
BitDefender
ClamAV
eSafe
eTrust-VET
eTrust-VET (BETA)
F-Secure
F-Secure (BETA)
Kaspersky
McAfee
McAfee (BETA)
Nod32
OneCare
Panda (BETA)
Sophos
Symantec
Symantec (BETA)
Sources:
here,
here and
here