Hmmm...
My first thought was that you are in "Cover View" too, which is what happens when you click on the that little 4-arrow icon on the far right end of the view header, but I'm not convinced that's the answer as when you go into cover view, the four arrows on the icon turn around to point inwards. The other clue to the cover view question is at the opposite end... if you're in cover view mode, then the task bar should also be, erm, covered!
So what else? Perhaps the Media Center window is not in a maximised state, but restored instead, and sized larger than your screen? To check that, right click on the Media Center taskbar icon (if you're using Windows 7 with taskbar thumbnails, right click on the popup thumbnail instead) and check if the "Maximize" option is greyed out?
If it is not greyed out, and you click on it, does that sort things? Then, if you click on the middle, 'restore' button, does it go all big again?
If yes, then get to that right click menu again, (while in "big mode") and this time, choose "Move". Tap the "Down" cursor key, then use your mouse to drag a corner of the program into view. Click the left mouse button to drop the window there. Now, use this corner to resize the program window so that it will fit in your screen, then exit the program to save the window size and position.
If I'm right, you should be good from there on. Please don't ask me why that has happened to you, coz I don't know!! Do you perhaps play any games, or run any other application that changes your screen resolution on the fly while listening to Media Center?
-marko.