You can use the free Partition Wizard boot CD to Move/Resize the left side of the C: partition into that unallocated space, have a look at Step Three
of this tutorial linked below for an outline.
Just ignore the first 2 steps.
Partition : Recover Space Used by an Older OS
As good and reliable a program as Partition Wizard is, I would not trust the installed version for the operation you need, use the boot CD as suggested and be sure to post back with any further questions you may have and to keep us informed.
First download the Partition Wizard Bootable Disk (PWBD) ISO file to the desktop, be sure to get the PWBD and not the installed version; scroll down to see the download link for the Bootable CD ISO file at this link below.
Partition Wizard Free Bootable CD
Then use ImgBurn to burn the ISO to a CD, at no greater than 4x speed with a verify; it was designed for use and works best from a CD rather than a DVD; then boot
the created CD to make the changes, it doesn't take any input from the user to setup, just relax and let it load.
ImgBurn Free ISO Burning Software