Sounds like you may have a corrupted system. Your machine will take up to system 10.5.8. Boot from the system installation disk, go to the second screen pull down menu to Disk Utilities. Select your boot drive in the left hand column and see if you can repair the disk. Next repair the permissions. That failing, copy your documents to the other hard drive then reformat the hard drive using the "write zeros" option and reinstall your system.