Computers today are not distributed with a Windows disc. Instead, the Windows backup (and other stuff) is put on a small partition. This partition normally carries the letter D. Windows sees it as a drive. But it’s on the same hard drive as C.
If that is your situation, leave the D drive alone. It should be very small, and have little open space. You should also copy it to a DVD. You’ll need the DVD if the hard drive fails. In that case, the D drive would be inaccessible.
There is another possibility. Your hard drive may have been divided. Sometimes, techs like to put Windows and other programs on C. D drive is then reserved for personal files.
Personally, I think that causes more trouble than it’s worth. I’ve had many complaints about that situation. The C drive eventually fills, while D has a surplus of space.
You can change these partition sizes with a third-party program. They don’t destroy data in the process. Windows also can change partition sizes, but it destroys data. This is not something that most people would want to take on.
If you have a surplus of space on D, move programs there from C. You’ll have to reinstall them; otherwise, Windows won’t know where they are. Try to avoid moving partitions, unless you’re handy with a computer.
You have options. All it takes is money. The easiest: Add an external hard drive. You can get an external terabyte hard drive for about $100. A 2TB system will run around $200. Most external drives connect to your computer via USB.
You can also install a second hard drive in your computer. That’s handy if you’re short of real estate on your desk. That will cost you even less. But it’s more difficult, of course.
Here’s more information on this topic from Kim Kommando's web site: