I have 2 hard drives xp and Windows 7 on my computer, only windows 7 shows at start up how can i get XP to show up to boot to.

I have a computer with windows 7 on it. I have an old computer that does not work with windows XP on it.  I put the XP hard drive in the new computer and want to choose which system to start at start up.  How do I do this, and please give step by step instructions.