Simple. Download the free demo version of Boot-It NG (
http://www.terabyteunlimited.com/bootit-next-generation.htm), which is an exceptionally good partition management tool (Well worth buying and keeping in your "toolbox"); and use it to do the following:
(1) Resize D: to free up some space
(2) "Slide" D: to put the free space adjacent to C:
(3) Resize C:
Done :-) You can use this technique to make C: as large as you want.
Here's a bit more detail on the process:
http://www.experts-exchange.com/Storage/Misc/Q_21929035.html... note in particular the illustration in this comment:
http://www.experts-exchange.com/Storage/Misc/Q_21929035.html#17169507