Run Auslogics Disk Defragger on C, you will likely see System Files at the end of the partition blocking further shrinkage.
Next backup your files and a Windows 7 Backup Image, boot free Partition Wizard bootable CD, rightclick C to Resize, drag right grey border to the left to see how much it will allow you to shrink. It should allow more since the OS files will not be running and can be moved easier. Free Download Magic Partition Manager Software - Partition Wizard Online
If PW CD cannot shrink it as much as you need, next I would install free Perfect DIsk 10 trial version, defrag for Compactness, then rightclick on C to Offline defrag System files. Now try shrinking with PW CD.