Hello,

I would highly adviced you to use public transportation in Madrid. It is very good and perfectly save and there's a subway station at the Parque Ferial Juan Carlos I (campo de las naciones), so you can stay at the city center and be at the fair within minutes. If you'd like to make short trips to towns nearby Madrid (El Escorial, Alcala de Henares), you can still use public transportation (trenes de cercania), and even if you are planning to visit Toledo, Avila, Segovia you can still use "regionales" trains.

check these URLs for further info:

http://www.metromadrid.es/
http://www.renfe.es/cercanias/madrid/index.html
http://www.renfe.es/regionales/

Un saludo,