How do I convert my Mountain Bike into a Road Bike for very little money?

I signed up to do a charity ride this year and needed a cheap way to convert my mountain bike to a road bike. I did some research and found the Avenir Streetster Tires. There were the best deal around. They get you up to 80 psi which some of the other cheaper slick tires do not do. I also had to buy additional tubes to fit the 1.5 inch width but they were only about $5 each.