I switched from AVG to Avast about 6 months ago, and have never looked back. It is also free, but gives you a LOT more options that AVG's free version. You can set certain folders, files, drives and such not to scan. I hated the fact that AVG insisted on scanning every drive on my system, even the backup drives that rarely get used.
One word of caution, change the .wav file preferences when you install Avast! I found out the hard way when it woke me up at like 3am stating "The Virus database has been updated!" (5.1 speakers set at a fairly decent volume level, as I use Home Seer as my alarm clock). Of course it woke me up!, I forgot to change it the next day, and this went on for like 3 days hahah!