start > right click "computer" > select "manage"
you'll see "storage" and inside "manage drives"
shrink your c patrtition and create new one in empty space. it will take some time obviously.
if this won't work for u then use gParted (http://gparted.sourceforge.net/
it's free and good. comes as bootable disk image. can be usb or cd (good to have just in case