Eu não tenho experiência nisso, mas o seguinte artigo da eHow parece promissor:
Using a touch screen with a computer allows you to do some things you wouldn't be able to do with just a mouse and keyboard. You can interact with the computer in a more natural way. You can also make sketches or take quick notes. Converting a liquid crystal display (LCD) to a touch screen can be done by installing one of many kits available. These kits contain a clear screen overlay to put over the monitor, a screen controller and universal serial bus (USB) or other cable type that plugs into your computer. The touch screen overlay is the same size of the computer monitor and is installed over the monitor in the monitor case. This is a fairly simple upgrade that most people can perform.
Things You'll Need:
- LCD touch screen conversion kit
- Microfiber cloth
As instruções são explicadas aqui: Como converter um LCD em uma tela sensível ao toque
Você pode pesquisar no google por esses kits de toque. Eu me deparei com isso , por exemplo.