Do you have multiple partitions on the same physical drive with C? If not, you have to buy a larger drive. But, if you do have several partitions on the drive and also has some free space there, you can try to add the free space to your C drive with the help of some free partition resizing tool: http://www.resize-c.com/
Of course, if the free space is continuously behind this C drive, you also can extend it in Disk Management directly.
PS: Do not forget to back up all important well before resizing your drive in case of data loss.