This is a Server OS and avast 4 Home/Pro won't work on a server, it requires the Server version of avast (even if that would work with an early release of windows 2008)
avast probably won't work with it in its current form, when Vista was in beta (and this build you are talking about isn't even there yet I don't think). A lot of work had to be done to avast so it was compatible with Vista and this didn't fully stabilise until the Vista RC version releases.
So if you aren't posting on the incorrect forum (not avast 4 server version), then avast 4 home/pro won't work on a server OS platform.