5308 install boots into existing OS

I'm trying to install Vista onto a system with an existing Win2K Pro
installation.

I set the PC to boot from CD/DVD, put the Vista DVD (burned from the Feb
CTP ISO image) into the DVD drive. The display shows the BIOS messages,
then indicates that it's booting from CD/DVD and there is a lot of
activity from the DVD drive but the display doesn't change for a minute
or two. Then, the Win2K OS boots up. Once Win2K has started, the DVD
drive is not present in explorer or device manager.

Is it Vista that for some reason is causing Win2K to boot? What can I
do to stop this happening? Before anyone suggests it, I don't have a
spare clean machine, it has to go into a new partition on my existing
system.

Thanks,
Ian