avast! installer already contains 32bit and 64bit binaries. Installer detects what OS you afre using and installs the required files. In the end, it doesn't really matter what you're using, avast! will make a decision instead of you and install the correct files. And yes, avast! is 64bit. Otherwise it wouldn't work on 64bit operating systems and it clearly is working on those. So yeah, it is 64bit.