Although the Install template doesn't contain the server protection package and the clients' installation still result in the mentioned problem, please check if the CMS version is 4.2 Build 34 or newer.
To do this, run the VirusBuster 2006 Console (not the MMC version) and find the version number in the Component versions window on the main page. If you use older version than the above, you should update the CMS with the help of Updater.
(The same thing happens when the CMS installs or updates the server program on a client. In such a case the CMS disables the installation for the lack of the Professional products. The solution is the same.)