Sure - you can do dat.
Name your external disk with whatever letter you want - your music library will stay intact, since it doesn't care where it lives. iTunes does, however, so you'll need to tell it where to find your stuff. Edit>Preferences>Advanced tab - click on Change, and enter the new location (for example, E:\iTunes Music).
Good to go!
Edit - just thought of something else - have you plugged your external HD into your new computer yet? It's possible (make that likely) that it will automatically assign a new drive letter to it as part of the recognition process (You know--Windows has found new Hardware, yada yada). If so, you're ahead of the game.