If the D: is truly empty you should be able to go into the Disk Management, remove that drive then expand the C: drive.
Here is a tutorial to get you started:
http://www.bleepingcomputer.com/tuto...torial133.html
And, as always, when you are playing with partitions, make sure you have any critical info backed up. If you make a mistake doing these operations the data is gone.
And my time to create this post is more expensive than Trad 0 I will only charge you a Golden Tee machine (which, if you think about it, only cost you $25, so it solves your problem for $25 - What a deal!)