Your system may be corrupted or your hard drive failing.
Try starting up from your system installation disk. Insert the disk and restart holding down the "C" key. Go to the second page pull down menu to Utilities, Disk Utility and see if you can repair your hard drive.
Try starting up from your system installation disk. Insert the disk and restart holding down the "C" key. Go to Utilities, Disk Utility and see if you can repair your hard drive.
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Try starting up from your system installation disk. Insert the disk and restart holding down the "C" key. Go to the second page pull down menu to Utilities, Disk Utility and see if you can repair your hard drive.
Try starting up from your system installation disk. Insert the disk and restart holding down the "C" key. Go to Utilities, Disk Utility and see if you can repair your hard drive.