Through windows disk management tools you can only shrink or expand the tail end of a partition.....in other words you cant shrink the beginning of D with what you have.
But you can use Gparted to shrink D on the front side of the partition...then go back to windows to expand C out.
There is one other problem though...many times when partition work is done outside of windows,it wont boot and you will have to do a start up repair....to do this you boot your vista dvd and choose start up repair.
http://sourceforge.net/projects/gpar...d-live-stable/
choose the iso.....then burn an image
boot the cd...shrink D apply the change...exit out
Its very easy once you get booted to the cd....there are some keyboard prompts along the way before you get to the partition tool...once there however its very easy...complete with visuals etc