Unable to change size of drive C:

 I cannot change the size of my C: drive. The message I get is that I already have the maximum size (264.7 GB out of 320 - ??) or I get the message that there is no unassigned space next to the partition (sorry, the program is in German and I don't know the proper technical terms). I have no problems increasing the size of another partition. It is just the C: drive that refuses to cooperate. Is there something I can do? Thanks.