This is very easy to do with a newer operating system, but I'm guessing this machine is probably running XP.
Assuming there is nothing at all on the D drive that you care about or is in use by the system, right-click on My Computer and go to Manage, then Storage/Disk Management. You should be able to right-click on the D drive partition and delete it. You *may* be able to then right-click on your C drive partition and extend it into the unused space. If not let me know; there are numerous 3rd party utilities you can use to do that part.