I have a Dell Inspiron 9100 dual-booting WindozeXP and Gentoo 2.6 kernel, but my hardware is quite different. Just the most basic questions -- have you checked the BIOS configuration to make sure that you have boot from CD enabled? Also, do you have another computer on which you can try booting the same CD to ensure that the CD is not corrupted? My guess is that the image is corrupted and you might need to burn a new one. Gentoo forums have good instructions for creating bootable CDs.
You can also try creating a Windows "rescue" CD (can't remember the procedure for XP, but you should be able to find it at www.annoyances.org
) and see if that boots from a cold start to test basic "boot-from-CD-ability" if you don't want to mess with the BIOS settings at this stage.
Good luck and let us know the outcome.