Back up your data. Run disk utilities from your OSx install disk. That should tell you the general health of your disk.
If disk utilities gives it a clean bill of health format the hard drive using the "write zeros" option. The write zeros option will map any bad sectors on the hard drive so that no information will be written to them and be lost or corrupt. Install your operating system again.
If disk utilities says your hard drive is on it's last leg replace it.