Here's a simple explanation of how to do it:
http://www.nirmaltv.com/2009/05/12/how-to-resize-disk-partition-in-windows-7/Unfortunately since you want to move space from D to C I think you'll have to delete the D partition, expand the C partition, and re-create the D partition in the remaining empty space. Alternately you could just delete your D and expand the C to fill the entire drive. You'll never have to worry about it again, just make sure to keep everything defragged.
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