Actually, there is a way to dual boot Vista x64 and XP x32 and retain
same drive letters both under XP and Vista.
Here is how:
Under XP change the drive letter of your DVD-ROM to something other
than the drive letter you want to have your Vista installed.
Boot from Vista DVD, click "Install Now" and hit Shift+F10, start
diskpart and make any partition changes you need (I personally shrunk XP
partition by 32Gb and then created another partition for Vista and
assigned it drive letter D: )
Go through Vista install as you normally would (make sure that
"Automatically activate when connected to internet" is unchecked so you
don't have to activate Vista twice -- you'll see the reason for that
later). Also don't install any updates or non-disk-essential drivers
Once you log in to Vista for the first time -- open to registry editor
and go to HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices. Then rename
\DosDevices\D: to something else for now (presumable this is your XP
partition), rename \DosDevices\C: to
[DESIRED_VISTA_DRIVE_LETTER] should be replaced with the desired drive
letter for Vista partition -- D in my case). Now rename previously
renamed \DosDevices\D: to \DosDevices\C:
(see 'How to restore the system/boot drive letter in Windows'
Close regedit and reboot. Now if you boot into Vista you will not be
able to run anything and your profile will not be loaded -- so instead
of booting normally choose Safe Mode with Command Prompt (by pressing F8
on boot menu screen while Vista option is highlighted). Once you boot
into Vista Safe Mode with Command Prompt change drive to your DVD drive
and start Vista setup AGAIN by typing setup in command prompt.
Go through Vista installation again and enjoy same drive letter
assignments under XP and Vista