Booting form vista and XP

I bought a computer with XP on it. Then due to security problems A friend installed Vista on it. i am not satisfied with vista and want to go back to XP the original operating system. I have two internal HD installed on my computer and I want to keep vista in one HD and install Xp on the second and be able to boot form either one. I don't know how to do this. I need assistance.

 

Simple, you boot up either OS and jot down the locations of the OSes, boot up XP disk, install XP in another location.

 

Easiest way to do this if you have both disks is to install XP over your current Vista thereby wiping it out and yhen installing Vista on a different partition other than C. If you want to install XP without touching where Vista is, then try this guide.