Use free Partition Wizard bootable CD to "Resize" C.
Partition Wizard.URL
If you still can't shrink enough, then defrag using Auslogics Disk Defragger to see what files are blocking shrinkage on the right side.
If these are System Files, use free Perfect Disk 10 trial program to first "Shrink to consolidate files", then rightclick C to Offline Defrag>System files.