You can install in on another drive, or just copy the Mathematica folder from
C:\Program Files\Wolfram Research\Mathematica
to D: or where ever. In the latter case, you will need to edit or delete and rebuild shortcuts (start menu, etc).
If you just move Mathematica, you should reset preference and cached files by holding down the SHIFT and CONTROL keys the next time you start Mathematica. (See http://support.wolfram.com/kb/12464 .)