Boot from the OS install disk. Once it reaches the Installer screen, check the menu bar. The Apple or File menu will give you to option to go to Disk Utility. Go there, and let it repair your hard drive. Then let it repair permissions on your hard drive.
If it finds no errors, but you still can't boot successfully, then it's time to reinstall the operating system. Use the option to preserve Users and Network Settings. This will retain your home folder unless you have set up as the root user.