When you “delete” files they’re not really deleted and they’re not taking up space either. Think of your hard drive as a chalk board. When you erase a chalk board, you can’t see what you erased (very well) but somewhere in the slate of the chalk board is everything you’ve ever written on it. But still you can write something new on it. So it is with your hard drive.
When you “delete” a file, you’re telling Windows that you don’t want that file anymore and telling Windows to use the space that file once occupied for something else. So Windows shows the space once occupied by that file as “available” so you can install a new program or use the space that whatever you deleted was occupying for something new.
But, way down deep on the magnetic surface of your hard drive the file that you deleted is still there. That’s how the FBI and other authorities gather evidence against criminals who think by deleting or formatting their hard drives they can erase all the incriminating evidence it might have once contained. But there is software available that can capture the faintest particles of deleted files and restore them. There is hardware available that can even extract more data from “formatted” hard drives. In fact most “formatted” hard drives can be completely restored. The software and hardware that can do this is very expensive for the most part. But you can find programs to download (some free) that can easily “undelete” a freshly deleted file.
The only way to completely remove data from your hard drive is by “erasing”. Erasing is a very misleading term. If you want to be sure that deleted data can never be recovered from you hard drive you need to use a program that replaces the deleted data with gibberish. An “eraser” program like “Eraser” replaces “free space” created when you delete files by overwriting it many, many, times with unintelligible data (usually random sequences of numbers, letters, and symbols). Some good eraser programs may overwrite it hundreds of times to make recovery impossible or nearly impossible.