I have a computer with space for only two hard drives. It was originally built with Windows XP Pro (Disk 0). I added a drive (Disk 1) and loaded Vista Ultimate on it and I do a dual boot routine as needed. At this point, I'd like to install a new Disk 0 and build it as Windows 7 Pro. (I really wish I could upgrade my Vista Ultimate to Win7 Pro) I guess my question is this, if I put in a new Disk 0, boot to a DVD and build it to Win 7, what will happen to my Disk 1? Will Windows still recognize that there are two bootable drives and ask me which one to use? When I put a new drive into the disk 0 spot, the system didn't seem to know what to do which leads me to believe that there's something in the MBR of disk 0 that tells the system that there are two bootable drives. I guess you could say that I'm not sure exactly how it all works. Thank you for any help.