. By my confidence in Avast has been severely shaken.
these rogues are not easy to detect.....but are a malwarebytes speciality

Fake antivirus overwhelming scanners
http://www.networkworld.com/news/2009/100209-fake-antivirus-overwhelming.html

you should also remeber that Malwarebytes is a specialised tool with only a bit over 300 000 signatures while avast are in the millions, so you need both. avast will detect lots of stuff MBAM will not