Scratch

All you need to turn your computer monitor/projector screen into a touchscreen costs less than $50! You'll need:

A Windows computer. Sorry Mac.

A Wii Remote (No Wii needed!) $20 (Wii remote plus will work, but there's no benefit.)
An infrared pen $10-20 (If you're good with electronics, you can rip out the internals of an Expo marker, put in an infrared LED, and attack a battery and button for about $5, I bought one that is high quality.)
A Bluetooth dongle $10 (Some computers have Bluetooth built in.)

Here are the steps:

1. Install your Bluetooth dongle if you haven't already.
2. Hold down the 1 and 2 buttons on the wii remote.
3. Click the little Bluetooth icon on the taskbar and click Add Device
4. Select Nintendo RVL-CNT-01 or something like that.
5. Download Pentabulous from www.wiimoteproject.com/pentabulous
6. Mount your Wii remote to the side of your screen. It has about a 45* angle of view from that black spot on the tip.
7. Start Pentabulous and calibrate the screen by pressing A on the wiimote.
8. Turn on the pen on each of the spots.
9. Now control the mouse using your pen!
10. Make sure that the wiimote is as close as it can be while still getting the entire screen, the farther it is, the less accurate it is.