Make Old XP Boot Drive Work on New System

My computer died several weeks ago. I bought a new Dell system with Vista. The old computer’s hard drive running Win XP is fine. How to I make the old drive the new computer’s boot drive. I have hundreds of programs on the old drive that I don’t want to reinstall – probably can’t even find the original disks. I tried just putting the drive in the new computer, but the system won’t boot.