Adjusting partition size

I have a Dell Inspiron 1764 with Windows 7,  which came partitioned into a C: drive for the OS and a D: drive.  The C: drive is about 60GB and the D: drive is about 400GB.  I am wonderiung how to repartition these drives where I can extend the partition of the C: drive by 125GB.  I have tried using the Computer Management Program in Win 7, but I don't really know what I am doing.  I have shrank the D: drive by the amount I wanted, and tried to extend the C: drive by the free space that is created, to no avail...any suggestions?