I have ubuntu 11.04 64 bit.
I tried "sudo dpkg -i avast4workstation_1.3.0-2_i386.deb" and it gave me the following message
error processing avast4workstation_1.3.0-2_i386.deb (--install):
package architecture (i386) does not match system (amd64)
Errors were encountered while processing:
avast4workstation_1.3.0-2_i386.deb
I ended up using "sudo dpkg -i --force-architecture avast4workstation_1.3.0-2_i386.deb" on suggestion from IRC as something to try.
I tried it, and while it worked to get it installed, it would not update.