A third party application might be running in the back ground while the volume is in use. 
Windows won’t allow any partition or resizing to be done while there is a program running in the background.
Or Windows is just not allowing any changes to the C: drive while booted to Windows.

To do your partitioning, re-boot onto the Partition Commander 10 CD.
When booting from the CD, select VCOM Linux, after select the top option 'Partition Operations', you can do the partitioning from here; all the necessary partitioning tools are available from the CD.