Don't right-click the Unallocated Space ... right-click your C: partition - the assigned space to the left of the Unallocated Space.
I have recently fitted a 250GB hard disc in place of the original 80GB, in my Dimension 3100c. I then loaded the new disc with a cloned/image of Win XP taken from my original disc. To my surprise the system booted up and worked ok. The only problem I can see is that the system has NOT recognised the larger capacity disc. When I go to My Computer / Local Disc /Properties the disc is still the original one.Is there some way to make the system see the new one?  Any help would be greatly appreciated.  
Fitting Larger Hard Disc