If your computer is running out of hard drive space, it can cause a number of problems. You may not be able to add programs, since there isn't enough room left on the drive. It can also slow down your computer, because there's more stuff on it for the operating system to search through. In addition, your PC occasionally uses your hard drive like RAM, temporarily storing data on it (this is known as "paging") for a program to retrieve quickly. If you don't have space on the drive, it can't be paged, which can further slow down your machine.
Windows includes a program called "Disk Cleanup", which finds data that may be unnecessarily clogging your hard drive, and deletes it (with your permission); this tutorial will take you step-by-step through Disk Cleanup, and how to use it.
First, click on the "Start" button, and type "disk cleanup" in the bottom search window. You'll see "Disk Cleanup" at the top; click on it to open.