I suppose you want to shrink the C partition to create another one, is that correct?
If so, you can use the disk management feature, right click on the partition you want to shrink. The problem you might be facing is that Windows 7 only allows you to shrink it up to the point where unmovable files are located. You have to defragment your volume to be able to shrink it further.
You can also use a third party program such as Gparted to do that. It will move your files to make room for shrinking the volume
Anything yuo decide, please please please BACK UP your data first. Things can go very wrong when playing with partition sizes and you can loose all your data.