The easiest thing to do here would be to just make an iso of your physical media. dd works well... for example, if /dev/cdrom is your cdrom drive then:
# umount /dev/cdrom
# dd if=/dev/cdrom of=windows2000.iso
This will make a "windows2000.iso" file in whatever directory you are in.
Edit: I'm more a vmware guy than xen, but last I checked booting the guest from physical cdrom wasn't supported yet. Maybe it is now... either way, I think ripping the cd to ISO is the easiest method overall.