Help: Dual Booting XP on Vista Computer

My Tiny desktop which ran Windows XP has failed. I have a Reload Disc and the XP Product Key is on a sticker on the desktop case. (Tiny no longer exist).

I want to convert my Toshiba laptop, currently on Vista, to dual-boot to XP or Vista so that I can use many essential XP files.

I understand that the hard drive must be partitioned to allow the addition of XP and roughly understand how to do this. Can I then install XP using the reload disc? Does the Product Key help? Or do I have to buy an XP Installation Disc?

This must be a very common problem. It is a disgrace that Microsoft do not make it easy for users to make this conversion. Backward compatibility is an obvious and key user requirement. Detailed advice would be welcomed.