As you are well aware since you have a 64-bit computer (and supposedly a 64-bit Operating system such as XP 64-bit or Vista 64-bit) older applications (software) were developed for the 32-bit systems which were and still are the standard. However with the growth and development of 64-bit Platforms, software vendors will develop software for it.
In this case avast offers the same protection on the 32-bit and 64-bit with the exception of the boot-time scan. When Version 5.1 will be released, the boot-time scan will be avvailable on the 64-bit architecture as well.