The most common way to install ISPadmin is using installation DVD which is available at promo events, or downloadable from download.ispadmin.eu. It is a ispadmin-XX-64bit-debian7-DVD.iso file (XX represents ISPadmin version).
ISPadmin runs on a Debian GNU/Linux distribution which is included in the installation DVD so there is no need to have a Linux installation ready prior to ISPadmin installation. Just burn the downloaded ISO file onto a DVD and follow the instructions.