I used Boot Camp to install XP to a separate partition, and in order to boot to it, I hold the Option key as soon as my screen turns that white/cream color (right after turning computer on) ...then I get the 2 pics of the hard drives (representing the 2 partitions), one labeled Mac OSX and one labeled Microsoft Windows XP. It usually only takes 5-10 seconds of holding the Option key for the boot menu to show.
I just Googled your problem, and found again that you need to hold the 'C' key immediately after turning on the computer.
The only problem I can think of is that the disc you have in the drive is not bootable.