Hi,
I’m not an expert in this, but I think what you need is this:
http://www.avast.com/eng/adnm.html.

Otherwise, you may manually download the latest vps file from http://www.avast.com/eng/update_avast_4_vps.html and distribute a copy of it to every PC via your network, for example leave it on their desktop, so that each user double-clicks on it and this way updates his/hers avast!