Okay, so this has nothing to do with where iTunes is installed.
Instead, move the media from the C drive to the D drive by doing the following.
Move the ENTIRE iTunes folder to the D drive.
iTunes folder default locations are:
- Mac OS X: /Users/username/Music/iTunes/
- Windows XP: C:\Documents and Settings\username\My Documents\My Music\iTunes\
- Windows Vista: C:\Users\username\Music\iTunes\
- Windows 7: C:\Users\username\My Music\iTunes\
- Windows 8: C:\Users\username\My Music\iTunes\
Hold the SHIFT key and launch iTunes.
When prompted to create or locate the iTunes library, point iTunes to the new location.