Sounds like your harddrive is dead man, but I would go into System Preferences-Startup Disk and make sure Macintosh HD is selected as the Startup Disk.
If that is not the prob, pop in your Install Disk and open disk utilities in there, and verify disk, if its the harddrive it should state it in there, if your lucky like I was with this same prob, it could just be a software issue and you can repair with the disk.
My guess still remains that you just may have a dead or failing harddrive.