Have you tried starting up in Safe Mode and see if it gets to the desktop? You can do so by holding down Shift when you hear the startup chime. If it gets to the desktop fine in this manner, launch Disk Utility and repair permissions. Once that finishes, try rebooting and see if it starts up normally. If not, then boot from your installation disc and once you're at the welcome screen for the installation, go to the Utility menu and select Disk Utility (this is assuming you are using Tiger 10.4.x). Once Disk Utility launches, select your hard drive and then under the Disk First Aid section run a Repair Disk. If it fixes anything, run it again until it says that the hard drive is OK. Then restart from the menu and have your Power Mac startup as normal, checking to see if it starts up successfully.