The existing OS is irrelevant because it takes no part in determining what device needs to be booted from (otherwise you'd have trouble installing anything on a fresh drive!
). It is most likely that you have not got your BIOS boot order correct, which means that your system is going straight to the HDD to boot and ignoring the CD.
EDIT: Whoops, I read your other thread that sounded like a boot order problem but replied to this thread by accident. It would be handy to know the hardware specs of the system. If this is indeed the same problem that you described more vaguely in the other thread then it's nothing to do with boot order.