This is what I've done to give my music folder a drive-letter.
I've put the music on my external drive which is called the D-drive. On that drive I have the folder 'Music'. In that folder I've put, of course, all my music (in subfolders too, but that's not that interesting in this example)
I've made a Dos-batch in which I give this folder (D:\Music) the drive-letter R
It's quite simple:
@echo off
subst R: D:\Music
That's it. I stored this in a batch called ChangeLetter.bat and thát batch file I put in the Startup folder of Windows.
So now every time I start Windows, this batch is launched which give me the drive R.
Of course in MediaCenter all music is pointed to that drive (and its subfolders).
If you use that batch on both computers they will have the same R-drive, even if the original folder is located on different locations (say, at home in D:\Music and at the office in C:\Music).
Hope this will get you a bit further.