problem on installing ubuntu 8 server on dell r200 rack mount system

I was try to install Ubuntu 8.04 installation CD on to this Dell rack mount; model R200 with a CDRW/DVD Combo Drive with 24x speed system.
Initially the application comes up to the 'Install to hard disk' screen. It then boots the Linux kernel, I can then choose a language - English, and then I choose location - Australia, keyboard - American English. Then the install progresses to 'detecting hardware to find cd rom' then it comes up with:-

 ‘Your installation CD-Rom couldn't be mounted. This probably means that the CD-Rom was not in the drive. If so you can insert it and try again'

However, the cd was in the drive and when I hit try again, the message comes straight back.

The server is a brand new Dell rack mount; model R200 with a CDRW/DVD Combo Drive with 24x speed.

Does Ubuntu 8.04 support this model of Desll server? Any suggesstions of how to get around this problem??
Thanks