I doubt the [strike]XP[/strike] 2000 system that you are transferring will run well if at all. It will not have all the necessary drivers for your motherboard and hardware. A repair install may fix it. Make sure the other drive is disconnected if you do this - best to work with one drive at a time.
If you are able to get the transplanted drive to work, I'd just use your bios's boot menu to pick which drive to boot from. Worst case, if you don't have a boot menu, you could go directly into the bios and switch the first boot device to which ever drive you want to boot from. This will be easier than trying to get the boot loader to recognize two OS's.