'this package is of poor quality and could damage your computer'?
You get that warning because they have not been tested by Ubuntu, by Ubuntu developers, so they have no way of knowing what damage they may do to your system. If you have to install a program, get it through synaptic package manager.
Next, why do you think you need anti-virus programs for a linux system? Only valid reason is to scan a mail server. Are you running one?
The design of linux does not allow a downloaded program to run, unless you, the user allows it. You have to go out of your way to set it executable. As long as you do not do that, you do not need an anti-virus program.