I think the XP that came with your compaq is OEM, and the key will not work with a retail or other OEM CD. Most likey you'll need to startup the recovery console as stated by EricB or it can be found on your start menu/program file /
It's possible now that most manufacture are providing the recovery program on a hidden partition. There are ways to start it up too if the F10 key does not work for you or the program doesn't work. You can set that partiton active, it will boot because it's bootable, it has the nesscary IO.sys, etc files.