Can you boot from the original OS disk? If you can, you could try using Disk Utility to Repair the start up drive.
Put the CD in and restart while holding down the C key. When the computer boots, pretend you are installing the OS, select the language and continue. On the next screen there is a full menu bar. Click on Utilities in the menu bar and select Disk Utility. Next select your boot drive (on the left) and in First Aid, click on Repair Disk. Do it twice, just to be sure.