[troll]zomg! you have a recovery drive INSIDE YOUR CPU?! I don't even want to know how you managed to do that...[/troll]
Is your "D:" drive an entire drive or is it a partition? Can you please right click on your "My Computer" icon, select "Manage," go to "Disk Management," put it in fullscreen, and take a screenshot and post it for us? That'd be the easiest way for us to figure out what's going on and how to help.
On many systems the D drive was the recovery drive and was pretty much full.
If you want to redo your C partition to factory settings then you should have a boot CD of some sort or have a utility on the machine itself that will aide you in making the boot CD that will allow you to boot off it and restore the C drive from the D drive. In some cases you can do this from a BIOS boot as well.
I would highly suggest you determine what you need to save from your C drive and back it up. Then go to the manufacturer website and review their documentation in helping you restore your system.