I too would HIGHLY recommend MBAM!! It saved my life!!! Avast + MBAM = Amazing combination and I have been speaking to the MBAM admins and they confirmed that it will run smooth with Avast , without ANY conflicts (no known conflicts upto now!), even with its real time protection on ( Although some dont like the idea of having 2 resident protection software running at the same time) - I have been running it and havent had any issues so far..
Avast does a superb job keeping the computer safe and anything that gets past it would be detected by MBAM..
PS - perform weekly quick scans on MBAM and regular scans on Avast! to make sure the computer is clean