A laptop with a screen you can write on is called a touchscreen laptop. Like its name suggests, a touchscreen laptop is equipped with an LCD touchscreen that can receive input from a stylus or other object that directly touches the screen, allowing users to, among other things, write on the screen. A laptop with a normal LCD screen can be transformed into a touchscreen laptop with a touchscreen kit. These kits can be purchased online and installed at home, though doing so will void your laptop's warranty.
Purchase a touchscreen kit for your specific laptop model. Touchscreen kits are available with two types of screens: plastic and glass. A plastic touchscreen is very durable, but not as clear as a glass LCD. Make sure you purchase a kit that comes with hardware such as screws and plugs, otherwise you may purchase them from a secondary store.
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Disassemble your laptop's LCD per the instructions in your computer's manual. Set the parts in a safe place in case you wish to remove the touchscreen and replace it with the original LCD at a later date. You may be able to find your computer's manual on the manufacturer's website if you no longer have the copy that came with your machine.
Assemble the touchscreen kit per the instructions that came with it, then install it into your laptop the same way you would install a normal LCD screen. Consult your computer manual for specific instructions.
Turn the laptop on. You should be able to move the mouse pointer using a stylus or your finger; the mouse may need to be calibrated, however, so that it moves correctly.
Click on the start menu and select "Control Panel", then "TabletPC Settings." Select the "Display" tab, then click the "Calibrate" button. Touch the stylus to each marker as it appears on the screen to calibrate the system. You will now be able to write on your laptop screen.