If you put the update file into a shared folder in the network, I think you could update avast! from there.
Download the update file here: http://files.avast.com/iavs4pro/vpsupd.exe
Hope this helps. Wellcome to forums. 
Btw, I suppose you will only use the Home version is you are in Home/Non-commercial network… Otherwise, you can: buy avast! Pro or, better, try the Network version of avast. 