Avast and Avira are more reliable than AVG. I used to use AVG, but switched recently to Avast and found it to be less resource-hungry and to do a better job of detecting.
I upgraded to the latest version of Avira and it clogged up my machine. Uninstalled it and switched to AVG and have been very pleased.
I set AVG to run at 3 a.m. every morning, and it catches bad stuff both then and in real time.
Use it in conjunction with ZoneAlarm free firewall.