|Do you see a message to hit any key to boot from the CD? If you don't see that message then your disk is not bootable or there is something wrong with the hardware.|
If you are trying to use the Compaq restore disk and a full version key, that isn't going to work.
If your WinXP CD is not working then borrow a disk from a friend. The version MUST be the same type. retail home/ retail pro/ retail/media edition, etc. OEM CDs shouldn't need a key, if they work at all. You will need that key for a full version.