Hey igor,
I don't know if this will help, but I was having problems updating Avast after installing the latest version on my windows media center 2005 PC.
I have two connections under Network connections, (I just got this computer in Dec so I'm still learning about it) One is the actual connection I use, and the other is called a 1394 Net adapter which is not in use.
For some reason the new Avast was trying to update through the 1394 connection first, Which means it would keep trying to update with no effect.
The way I fixed this was to disable the 1394 connection, reboot, then re enable that connection. From then on Avast used the right connection and updated fine.