jricart wrote you a nice tutorial, how to calibrate, but if you cannot do that then the touch screen glass has failed,...which is a common problem on these devices, you can grab one or two on ebay for 40-80 USD. look for touch screen glass for Xerox dc 240/250/dc700/4110/4112 they are all the same. When you receive the glass you dismantle the touch screen itself, it's not that hard but there's a lot of screws to unwind,...just take you're time,...then simply replace the touch screen glass but first check how are the cables connected (if you turn it around it won't work),...I've replaced it multiple times, now I can do it in less than 10 min