Avast has been installed on a 64-bit PC that came with the 32-bit version of Windows Vista Home Premium pre-installed. I'm happy to report that the installation went through perfectly and daily updates take a matter of seconds.
I read elsewhere on the forum that beginning with Vista, the VRDB wasn't used with Vista, yet after the installation, both its (i) icon as well as the (a) icon appeared in the taskbar. Should that have been expected?
Note that I was surprised to see a 32-bit OS pre-installed on 64-bit hardware...