since your NOT in a domain the administrative shares that avast uses to install are probably not there even tho you share folders....
do this to check it:
go to computer management (rightclick on the computer on your desktop or in the explorer and click manage)...
open up SHARED FOLDERS and SHARES
there should be several shares, among them are:
ADMIN$ C:\Windows
C$ c:\
Also make sure when you set share permissions that you give FULL CONTROL to that username and not to EVERYONE.... remove everyone from that share really...
if you have shared the wrong paths with the wrong names, correct them and try again
if that doesnt work you can create an MSI installer package, which would be better in this case in my opinion because you dont have a centrally managed enviroment for the usernames/passwords....
ps: your posting should have been posted in this board:
http://forum.avast.com/index.php?board=10.0