If you're trying to dual-boot Windows XP and Windows Vista, don't install them on the same partition. Even if you install the second OS into a different directory from
C:\Windows, you'll run into problems with the second OS replacing files in
If you're replacing Windows XP with Windows Vista, you should be okay. (You may want to reformat
C: or at least rename
C:\Program Files before installing, if the installer doesn't do it for you.)
Be aware that if
D: is formatted for NTFS and you create a new user account in Windows Vista, it may not have access to all of the files on
D: until you change the permissions. This might not be an issue if you use Windows Easy Transfer to migrate your account.