It is not recommended to install ISPConfig on a system that is already in use, because ISPConfig has to modify your apache configuration and websites with e.g. PHP that are in /var/www wont work anymore after the installation. They have to be copied in the new ISPConfig website directories before you can use them again.
Also ISPConfig does not install a new apache for your hosted websites, it will always use the apache that is already installed on your server.
Better try ISPCOnfig on another server or on a vmware or qemu installation on your desktop.