I use Avast myself, but I have had trouble with some of their shields that install. Avast is nice but bloated.
So every time I install Avast I do a custom install and I only install 2 shields, File Shield and behavior shield and nothing else. Avast runs smooth and still stops the viruses, and uses a heck of a lot less resources.
I have had trouble on random customers machines with avast and some of the other shields. On a few the web shield was not working right and blocking every single web site. On my own system the network shield was slowing my network to a crawl. Also with my email online I have no need for an email shield.
Ever since I turned off all the extra shields I didn't really need and turned off auto sandbox not a single customer has had trouble with avast, and it still catches the viruses since it still scans the files.
Just remember, NO AV is perfect and a lot of people get a false sense of protection. An AV can only protect you from what it knows. So proper habits and keeping java and flash up to date will make a huge difference. I haven't been hit by a virus in over 5 years. The last one I got was because of a hole in java.
Shane