How come I keep getting an error telling me I cannot delete a folder because it is in use?

Q: Can’t delete a folder because it’s in use. I tried deleting it normally, then with a file shredder, and then I checked the processes going on in the folder using “Process explorer” and I found a process. When I tried to “close handle” on the process it failed to work, giving this message: “Error opening process: The handle is invalid.” How can I get rid of it, it’s taking up a lot of space?