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 %ProgramFiles%.
If you're replacing Windows XP with Windows Vista, you should be okay. (You may want to reformat C: or at least rename C:\Windows and 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.
x86-64notx64. en.wikipedia.org/wiki/X86-64 – Brad Gilbert Jul 23 '09 at 0:35