I would suspect your hard drive is failing.
Option #1 You could just boot from the OSX disc and run Disk Utility to check the drive's SMART status. That may or may not show a problem with the drive.
Option #2 If you have a spare USB hard drive you can connect it and boot from your OSX install disc. Install the OS on the USB drive which you should be able to then boot from. From there I would download a free program called SMART utility and see if that shows the drive as FAILING.
There's a possibility the OS just needs to be reinstalled but you really don't want to try reinstalling it until you know if the drive is OK first.
Hope you have backups :-)
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