I am having a very similar issue today -- whenever I attempt to clip an image from a browser to Evernote, Avast quarantines it (claiming it's infected with VBS:Malware-gen). Unfortunately, this process corrupts the Evernote database (preventing the app from running), so the only way to regain function is to restore the entire database from a backup. Fortunately, I run Time Machine, but this could be a very ugly loss of data otherwise.
I was curious after encountering this issue, so I initiated a full scan. It is showing hundreds of infected files, also with VBS:Malware-gen. Given that I've been running all shields continuously for more than a year (and started with a clean scan), either these are false positives, or the Avast shields allowed my machine to become massively infected. I'm not even sure this malware is capable of running on OSX, my understanding is that it can't.
Any help would be appreciated.