Answer:
To use your computer screen as a touch screen, you screen must be touch capable.
If your laptop did not come with a touch screen, you will have to buy a new laptop with touch screen capabilities.
If your desktop monitor does not have touch-screen capabilites, you must buy a new touch capable monitor and connect it to your computer. You must also make sure that your computer is capable of processing touch gestures (anything after and including Windows XP, but no Mac).