I have Viewsonic touchscreens which use a USB connection for the touch data and a DVI connector for the video. Whenever I have had the problem you describe, it is as a result of the USB connectivity being interrupted. If unplugging and replugging the USB cable doesn't resolve the problem, you may have to exit Hauptwerk and reboot the computer.
Another possibility is that if you are loading Hauptwerk automatically when the computer boots up, the touchscreen driver may not be fully loaded before Hauptwerk starts loading. You can try adjusting the "minimum time to wait on start-up" under the general settings menu. The default is 0.
Also, you can try recalibrating the touch screen using the windows device manager options.
Ray