Repartitioning the C: drive is playing with fire, especially if you have an OEM install that did not give you recovery system CD's. It can be done with the right software and if you know exactly what you are doing. If you don't you may crash your system disk.
Make sure you first clean all the accumulated crap that builds up on C drive like cookie tracking, and temp file downloads. Then defragment C drive.
If still not enough then it may be time to do a clean install to start over. Windows needs a new install every year or two to clean house.
If you are dealing with an OEM system install that did not give you system CD's then check if there is a method to build a recovery CD first. Problem is some of these recovery CD's give you the original partition back again. Expanding the C partition before you put anything on D is much safer. Just deallocate D partition, expand C partition, and finally create a new D partition in remaining disk space.