if you DONT want to back everything up and do a fresh intal
boot from the lepard disk (instert disk reboot and hold c key) when you get there pick language and than i the menu bar under i think utilities go to disk utiliti find your hard drive and erase. go back instal and instal.
OR if you dont want to loose aynthing boot from leopard disk and just do an upgrade... you will have it all good to go and nothing lost
personaly if you want to keep anythig after an OS upgrade id sudgest backig up.it's crazy to trust an os to keep everything in tact