welcome to the forum.
yhea avast have some problem with that kind of malware but no antivirus program have 100% detection.
malwarebytes is a good program to deal with this kind of infecion.
http://filehippo.com/download_malwarebytes_anti_malware/
if you have the sample of that nalware you can send it in to avast by add the file/s to the chest and upload it from the chest to the avast lab so they can improve there detection.
hope this will do for you
welcome here agian.