Try right clicking your mouse over your normal desktop in a blank area. This will bring up a dialog box. You want to click on "Properties."
In this box there are several tabs. Choose the one that says "settings." In there you'll see a bar for adjusting the screen resolution. Look at what your resolution is set at now, then push it to the left of that setting and you should lower the resolution.
Chances are that somehow your resolution is set too large for what your monitor can display, so it solves the problem by showing what it can and letting the rest do a virutal "overlap." You could do as was suggested above and reduce the size of the browser window, but that won't help you with your desktop itself.