During the initial post you'll usually see options like 'Delete - Enter BIOS' or 'F4 - Network Boot'. There's also usually an option that says something like 'F8 - Boot Menu'. If you do see something like that then press the corresponding key.
That should bring up a menu loaded with all the devices that you can boot from. Select the optical drive that has your XP media and see if it works. And watch your drive lights to see if it's trying to access the media.
Also, maybe check the XP disc to see if maybe there's some fingerprints or something else on the read surface that's keeping it from being read properly.