Good idea to increase 'C'
Download EaseUs Partition Master
http://www.easeus.com/partition-manager/epm-free.html
At the Easeus main screen, Rt click on the pictorial representation of ‘D’ and choose ‘Resize’ Do so by moving the slider on the LEFT of the bar representing the partition to display the amount you want to increase C by.
This will create unallocated space physically next to C to allow expansion. OK the re-sizing. Expand C into the unallocated space. OK and Apply. Your machine will shut down to allow the expansion of C before it fully boots into Windows.
Remember to back up any sensitive data, no guarantees of success, but I hasten to add it's never failed me yet...