Why not go with one of the many other trusted (and often free) versions of Antivirus, like Avira Antivirus? How old is that disc? Chances are it hasn't been updated in years hence the "Not trusted" part of the warning. Of course if you're THAT desperate to install it, just right-click on the installer file (open the disc in Explorer window) and run it as an admin. Or even disable User Account Controls until it's installed. Though again, I urge you to consider other alternatives and to be cautious if you do go through with this installation.