If you have just bought a new computer already pre-installed with Leopard then you can't normally go back (check the Apple knowledge base for a list of what computers can go back to what revision - sorry I can't be bothered to look for the link for you.) If your computer came installed with Tiger (or earlier) then it should just be a case of running the install disc.
However I think you should get to the bottom of why nothing is working, it should. This isn't Vista.