Download and install Easeus Partition Magic (Home edition free) - Easeus Use it to delete or shrink D: and increase C:. If you have any data on D: you will almost certainly be in danger of losing it so back up first to be sure that you still have it afterwards. I would also back up any important data on C: just to be on the safe side. Program is quite easy to use and virtually holds your hand as you go through it.
Just check before you do use it that your Windows installation is on C: and that is is not just a restore partition to be used if your computer has problems.