Well there is always the chance that the problem is a short or a loose wire. Some devices run the cables for the screen around the hinge
(J600 and 700 series for example
) with excessive movement of the screen the hinge can rub or worse and cause problems.
However the most likly cause is either that part of the digitiser mambrane is trapped or just plain damaged. If you can get the front of the screen off, give the screen a good wash down with an LCD cleaner
(look at the hp recommendations first though before purchasing anything
), very carefully. Check the connections etc. Look for anything trapped between the protective layer and the digitiser.
Also there is a chance that your stylus has worn to a point and has scratched into the membrane. Please check that and if needed investigate a new stylus.
Lastly if you get it sorted, look to invest in a screen protector. They're woth it.
http://www.hpcfactor.com/reviews/editorial/protecting-your-hpc-2002...