You can test the touch screen using the app Xperia diagnostics:
https://play.google.com/store/apps/details?id=com.sonymobile.androidapp.functiontester
If the touch works fine in this app you could try to repair the phone software using PC Companion:
http://www.sonymobile.com/update
In PC Companion, press start on Support Zone > Start on Update the phones software > Press the blue link that says "Repair phone".
If the problem persists, you should have the phone examined by a service center. Please contact your local Xperia support team for more information:
http://www.sonymobile.com/global-en/support/contact-us/
