Dual booting Vista and XP (But not on the same hard drive)

I don’t actually want to dual-boot with vista, but I attempted to move XP to a different hard drive (it was new, and much bigger. I wanted to use it as the primary drive), and I now can’t boot into it, so I installed Vista RC1 on the master hard drive, expecting it to pick up XP, but it didn’t.
When I boot, it doesn’t give me a choice, it just goes straight into Vista. I’ve attempted to use BCDedit to add XP (D:) onto the startup list, but when I try to boot into XP using the boot manager, it just restarts the computer.

I’ve looked around for anything about dual booting, but they all use partitions, rather than seperate hard drives.

The reason I installed Vista rather than XP is because XP won’t let me install it any more. It says the key is invalid whenever I try to install, but the key *used *to be valid, and I’m using the same disk. I even tried using a pirated key, but that didn’t work either.

So any help on either of these issues would be greatly appreciated.