Probably the best course of action since you say you already have an external hardrive is to swap out the old HDD temporarily with the present external HDD and transfer the files copy/paste to the Vista machine.
Once you safely have the files transferred you can then safely formatt the HDD while its in the external HDD carriage, then transfer the original external HDD back.
At that point you can safely add the old HDD to the Vista machine as an additional data storage without the problems of a conflicting boot record because you wiped that out when you formatted it as an external HDD.
The worst option would be to add the HDD as a dual boot since it obviously contains another operating system and if it happens to be WinXP will probably cost you your Vista restore points in the dual booting process.
You can buy cheap external hard drive cases to install your drive in. There are two kinds of hard drive used mostly for home pc's. Sata-ata and ide. Sata has slits where the computer cables plug onto and ide has pins. It says it on the hard drive too. Just make sure that you but the right drive. After you assembly the eternal hard drive.. Plug it in usb.. should be listed under my computer next to your c drive.. you will see it.. Navigate through your documents and setting to your user profile and copy what you need.