What distro are you using? I would answer more, but I need to know specifically what distro you are using in order to further assist you. Note that if you are using ubuntu, here is a quick guide - http://www.ubuntugeek.com/avast-anti...u-desktop.html
Edit - Just realized something... If you are trying to install it for a normal computer, you need to download avast4workstation, and not the server version.