First off I think your C partition is a bit mean, you'll need more space than that! You can direct all your downloads to D as they arrive using the 'Save as' option in the window at the bottom of your screen. I manage to run Windows on a 64Gb SSD but I have to be careful. 100+Gb would make life easier!
Use Easeus Partition Manager to extend C to a manageable sizehttp://download.cnet.com/EaseUS-Partition-Master-Home-Edition/3000-2248_4-10863346.html