Ubuntu 11.10 64 bit
This is what I did to get the i386 pkg to work, make sure that you go to the registration page http://www.avast.com/registration-free-antivirus.php to get your key while your downloading the file.
I installed Aptitude for package management.
First you need some tool to edit/build .deb packages. GoTo the terminal and enter;
sudo aptitude install build-essential dpkg-dev fakeroot
Download avast4workstation_1.3.0-2_i386.deb and open the directory where you saved it to, mine was in Downloads. Then back to the terminal;
cd ~/Downloads
mkdir avast4
dpkg-deb --extract avast4workstation_1.3.0-2_i386.deb avast4
dpkg-deb --control avast4workstation_1.3.0-2_i386.deb avast4/DEBIAN
nano avast4/DEBIAN/control
With nano open using your arrow keys go down to Architecture: and over to i386 delete and change to amd64.
Save +
Exit +
Then enter in terminal :
dpkg --build avast4
sudo sysctl -w kernel.shmmax=100000000
sudo dpkg -i avast4.deb or double click on the new .deb package.
Go to accessories and click on avast and enter the license key and then use.
You can also try this: https://support.avast.com/index.php?_m=knowledgebase&_a=viewarticle&kbarticleid=313&nav=0,1,58,67
So far it is working for me with no problems