I've used Avast! On a different machine and didn't really like it all that much. Granted the machine was old, but I found it a bit heavy on the resources.
Most of the time, I used Kaspersky Internet Security, it's detection is great, but in more recent times (particularly version 2010) it's a large system hog, and it's quite hard to navigate.
Now, however I use ESET, and haven't looked back. On the bright side, it's cheaper than KIS, and very light. I like it.
Avast! Is good though, it's free, you can't go terribly wrong with it.