There's no way you can install a full Windows OS on a slave drive. (Or you can, but that means switching the disk priority from BIOS every time you boot the machine and that's not what we want, eh?) So you have to install the two OSes on the same internal harddisk.
Here's a pretty good guide on how it's done (usually, Google can be much better place to ask than The Raven):
http://apcmag.com/5485/dualbooting_vista_and_xp
Note: It's making a presumption that you have a Vista Install disc handy to make the partitioning. If you don't, just run a Google search on "partition software freeware" or something similar to find a suitable alternative for the job.
EDIT: Checking further into the article, I just noticed that having a Vista disc is mandatory, as one needs to use Vista's bootloader. Another solution to that is to use GRUB, but that also requires a little third-party OS...
Then of course, there's the bonus door:
http://www.microsoft.com/windows/products/winfamily/virtualpc/default.mspx
