I have been playing with this some latley and found an interesting item.

If you install XP first and then instal Vista as the secondary OS (this is the easiest way):
1.) vista will set up the dual boot screen for you and it should work
2.) Between opperating systems the drives will change letters, so make sure you are careful to only have the 2 drives so you can make sure they are C&D or close to that. IE: in Vista, drive C will contain Vista & D XP, in XP drive C will contain XP and D Vista
3.) Vista sees those XP boot and windows system files and WILL NOT LET YOU change the drive letter of the XP drive. Sorry.

If you install Vista First, and XP second:
1.) The drives set themselves up so they are static. IE: in Vista, drive C will contain Vista and D XP. In XP drive D will contain XP and C vista, still.
2.) You have to use the Vista install disk to repair the vista install after the XP install breaks the vista boot info.
3.) You have to use a boot manager app such as EasyBCD to edit the boot files to provide the option to boot into either OS.
4.) You can install Vista, change the drive letter you want to put XP on, then install XP on it and the drive will be the one you want in Vista. (problem here is that usually this causes the XP and Vista drives in XP to not allow you to change either of their letters after XP is installed)

Why do I know all this? Because I tried to install the system so C: was always the OS I was in, and make my other windows drive something down the line like W: and put my other hard drives up next to the main OS one. Alas, I was defeated by the Windows is smarter than you mentality. I surrender. Hope this helps someone out there.