Dual booting Win 7 64x and Win XP with two already installed hard drives

Without the CD Key you can not do a proper install. I would be looking for my Key.

IF this is legal (it's not if the XP came pre-installed under an OEM license on a machine that you bought), you can try the following:

Remove the Win7 drive and attach only the XP drive. It won't boot; the low-level drivers are all wrong.
Boot from the XP CD and choose to do a Repair installation. This will try to fix said low-level drivers.
See if it boots. If it does,
Re-attach the Win7 drive. At this point you can boot either drive, but must pick which drive to boot from in the BIOS.
Add the XP drive to the Win7 drive's boot menu. Now you can set the BIOS to always boot from the Win7 drive, and pick XP from the boot menu if you want it. EasyBCD is a great tool for this.

Please, if the XP is not a legal copy, or is an OEM copy, either buy a proper copy or abandon this thread. I'm not accusing you personally, but unlicensed installs are not allowed on these forums. If you built both machines yourself, you should be golden.

Thanks for the replies, don't worry, my CD is as original as it gets. My cousin gave it to me, and it was only in a paper envelope with a cd key writen on a piece of paper. I lost the piece of paper, but now everythings OK, thanks for helping me.