This <a href="http://www.cyclotouch.com.au/index.php" target="_blank">Touch Screen Overlay</a>, from Cyclo-Touch, looks like a brilliant idea, and can breathe new life into any neglected desktop monitor.
There is single-touch, multi-touch, and something else. Watch the videos to see how simple it is to fit: The cost of them seems very reasonable, but I can imagine postage being the sting in the tail.
Touch Screen
Moderator: Claw