Need help shrinking hard drive volume

Hi all. I've finally came to the conclusion that I need help with something as I am getting quite frustrated. I'm trying to dual boot windows xp that has vista already preinstalled on it. The problem that I am having is that by the website that I am using to follow the instructions for the dual boot process, I am running into a problem with shrinking the volume feature on windows vista. I beleive that I am running a raid partition because I have two hard drives that came with the computer and it is only showing as one. I am receiving an error everytime I try to shrink the volume. When I right click on the drive that I would like to shrink to make room for windows xp, i am getting the error Request not supported. I've attatched a picture to this post that shows the error message. Any help would greatly be appreciated.