Savage, don't use that registry hack.
It's for fixing a system that has got broken (i.e. the letter used to be x:\ and somehow got changed to y:\)
If you use it for cosmetic purposes, because you'd like your system to be a different letter than the one that the OS assigned itself when it installed, you'll break the system.
As well as that registry entry, there will be literally thousands of others which are dependent on what the OS called itself when it installed. That's why the hack's provided, to bring a confused OS back to the way it was at install, so that all the registry entries agree again. If you change it, you'll have thousands of orphan entries unable to find things.
It is completely irrelevant what a different OS calls the partitions. There is no physical link between the partition and the letter, it's just a figment of Window's imagination. Look at the partitions with Linux and there are no letters.
If you want all of your partitions to be the same letters whichever version of Windows is viewing them, then you'll need to install them in a way and a sequence that achieves that result.
There is a downside though.
If you have Windows A installed on C:\ which sees Windows B as D:\ and you install Windows B as D:\ and it can see Windows A as C:\, you will have problems.
Several 3rd party apps, notably Adobe's, which will probably be on all of your OSs, (like Reader and Flash Viewer), have an annoying habit of putting files in C:\Program Files\Common Files even if you instruct them to install to D:\Adobe.
This means that both Windows A and B will have put files in the same place (the second overwriting the first), which will cause unpredictable behaviour at the very least, and might even crash your system.
If you have each booted system considering itself as C:\ and the other systems it can see as notC:\, then you won't get this conflict.
If (like me) your Vista is C:\ and XP is D:\ whichever system is booted, then you'll need to take other measures to prevent C:\ being visible when XP is booted.
Because of the problem of XP destroying Vista restore points, this is necessary anyway, so the Common Files conflict can't arise.
Tell us what you'd like to do, and we'll advise
a) whether it's a good idea
b) how to do it, if it is.