How do I replace OSX server?

Hi all,
Looking for some help please. I have an early Mac Mini with OSX Server installed. I'd like to pass it on to my son, but want to put the original OS back on the box first. Unfortunately I can't boot from the CD - the machine just spits out the disc when I try to boot from it with the 'C' key held down.
I'm assuming that OSX server is preventing an 'upgrade' to the desktop version?
I can't find the server CD that the machine was originally configured with, so can't use disk utility to erase the system disk.
Any ideas appreciated!
Thanks
Mark