I don't know about vista, but XP allows you to change the drive letter in disk management. Right-click "my computer" and select manage, then click on disk management in the left pane. If you right-click on your H drive you should see the option to change drive letters.
I have no idea how it will effect the rest of your installed programs as some registry setting may still have paths to your H drive after changing the drive letter. You should be able to change the drive letter back to H if you run into problems.
I haven't used nvidia cards in the past 3 years or so. Does their installer have any command line options? If they do, their may be an option to change the install directory.