If its not what 2k1Toaster said then it may be your drivers.....
Make sure your drivers are installed and working properly (im assuming your using windows xp here) go here to see:
Start > Control Panel > Performance and Maintenance > System > Hardware > Device Manager > (Then look for your thouch screen in the list, I'm not sure what it would come up as, right now my screen isnt put back in one piece yet, lol.
If it has a red 'x' or a yellow '!' that may very well be your problem.
Anways hope this helps
-Simy