You would need something like partition magic, just remember to take anything of the d drive before you do anything.
There is a downloadable CD called Hirens boot disk, it contains many programs including partition management utilities.
It's an ISO or disk image, you would download it and burn it to CD then boot from it.
I've just reloaded by old Benq laptop after replacing the hard drive, I did just what you ask and changed the c partion to the full 120gb drive.