Avast is perfectly good enough and in my experience causes less problems than AVG or MS Security Essentials. Add Malwarebytes for it's quick scan facility to pick up something you suspect has infiltrated the computer and Emsisoft Antimalware for a thorough clean up.
You don't say which operating system you're using. With a slow PC, Avira AntiVir is good and doesn't hog resources to the same extent as some others. With a fast PC, I'd use Microsoft Security Essentials. Although I prefer Avast to AVG, in my experience it does slow the computer down, particularly with large file downloads from the internet.