"Would I be able to add the XP drive from the previous computer as a slave to the Vista computer then use Vista's boot program to recognize XP on the slave drive?"
No. XP will not run after the move. While it may be possible to add XP to the boot menu XP doesn't survive such moves due to many registry entries and more. There are some that have done this but you need the full XP CD with the service pack you are running and you run what is called a REPAIR INSTALL on the XP OS.
Frankly this is fairly advanced so maybe some boot manager like Acronis Disk Director and an install like you read at http://www.windowstalk.org/dual_boot_vista.htm
Bob
My delima is I purchased 2 new computers to replace 2 decent XP machines. I did not know they would come with XP and Office 2007 as Dell site only said free upgrade to Vist/2007.
Anyway, I can't run ACT! or other periferal software that I on Vista so I am stuck as I need the upgraded hardware from the new machines to ACT! better.
Would I be able to add the XP drive from the previous computer as a slave to the Vista computer then use Vista's boot program to recognize XP on the slave drive?
I know that I can't partition and load XP starting with Vista, but wondering if this way would work as well.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic