If your screen has lost its calibration, but you can't access the preference settings on your device (something that happens more often than it should!) - then before a hard reset which will also delete all your applications, try to use Recal. It's a simple, but really useful solution for accessing, with one click, the screen calibration page.
Its options allow you to launch the screen calibration window by pressing one of your device's buttons repeatedly, so you don't have to use the touch screen to access it, and you don't have to perform a hard reset.
One of those little applications which can really save your skin!