steam.exe - bitcoin miner / malware not detected by AVAST.

I noticed week ago that my GPU is always on load at 90% even if im on desktop and not doing anything with PC. I found process called steam.exe that was running and using around 16% of my CPU. Funny thing as i don’t have steam and file was located in AppData for application XnView. When i closed the process my GPU went to IDLE. I asked Google about this and steam.exe is a malware BitCoin Miner. I scanned file with AVAST but program didn’t recognised it as a threat which it should. I couldnt find an option in AVAST to add it to THREAT / BLACK list so i had to simply delete it from auto start and file itself and then scan system with other anti-malware software. I thought that AVAST is good. Im using the Avast Internet Security.

You can report it here: http://www.avast.com/contact-us.php?subject=VIRUS-FILE