Hey guys, my computer's done something funny. Whereas before any programmes running would take up the whole screenright down to the start bar at the bottom, there's now a space nothing will enter at the bottom. Instead I get a thin strip of my background. I've tried opening several programmes, and they all look like this:

Does anyone know what the fwip is going on here, and more importantly how to fix it?
EDIT- Even when I opened VLC and tried playing a video on fullscreen mode, it didn't. It filled everything except the gap and the start bar below it. It's like my computer thinks its screen has shrunk...