before launching dpkg -i avast4workstation....deb , you should :
be connected as root
and you should be in the directory where the package is .
for example if the package was downloaded in /home/mysoftware ,
open a console ,
type "su" (press enter key)
type " cd /home/mysoftware" ( press enter key )
type dpkg -i avast4workstation.....deb (press enter key )
avast is installed