OK, I'm slightly confused about the numbers, because there is nothing like 4.1.512.
There's avast! 4.1 (4.1.501), and avast! 4.5 beta (4.5.512).
So, if you simply want to get back to 4.1, then yes - you can just put the downloaded installer to CD and install it. There's no Network Shield inside 4.1, so just ignore what I wrote previously.
However, if the betaversion doesn't work for you, it's quite likely that the final 4.5 won't work either - because there really won't be much difference between the final release and the current beta (and you are the only one having this kind of problem).
If you want to experiment a little, I could send you a link for the full 4.5 beta installer (i.e. something you can start on a system without avast!, and you can select the modules to install during the setup - no need to perform an update over existing avast! 4.1) - so, you could try if the new avast! 4.5 feature (the Network Shield) is really the cause of your problems (therefore, I was asking if it should be Home or Pro).