Dual Boot Going To Single Boot


I now have a dual boot setup - XP and Vista - on two separate SATA internal drives. XP on the original drive is shown as the "system, active" drive and Vista is on the added (second) drive. When I boot up now I get the Windows Boot Manager page and choice of Eariler Version or Vista. Vista is the default. I would now like to use Vista exclusively, delete XP and reformat this drive for storage. How do I make the second drive with Vista the active drive (and a bootable drive?) before I delete XP. I have read all of the general info and postings on dual booting but I can't grasp exactly what I need to do to avoid a non-booting computer. Thanks for any help.
I switched the drives but when I tried to boot I received a message "ERROR LOADING OS" and nothing more. Could I use the EasyBCD program "Reinstall the Vista Bootloader" before I switch the cables?

I'm hoping to do exactly the same as above as my XP drive is starting to go faulty and I just want to completely remove it from the system and go solely with Vista.

When you say "swap the positions of the drives" do you mean switch the Sata cables around? I'm not sure what the next step is then, do you boot the system into Vista and run EasyBCD folowing the instructions in the link you gave

Many Thanks in Anticipation