is the white square on the desktop or floating as a separate window? Can you use your PC and programmes OK, but it just looks rubbish?
If the latter I suspect that there is some sort of custom desktop which has been corrupted.
To change it right click anywhere on the desktop and choose properties. The choose desktop and swap the current one from anything on the list. Save and the background will change. If it is as I suspect the white square will also have vanished.