As others have stated, you really should be backing up to an external hard drive because at present you'll lose everything if your current internal hard drive fails.
Have a look at this tutorial, which explains how to use Partition Wizard to alter the partition structure.
Partition Wizard : Use the Bootable CD
In your case, you could easily reduce the size of your C drive to 100GB, and then either create a new partition or extend your D drive to the left to take up the unallocated space.
It will help us to help you if you upload a screenshot of your expanded disk management layout too.
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