How to Repair a Macintosh Hard Drive

For safety, you should go to Applications > Utilities > Disk Utility, click your start-up disk in the sidebar and click Verify Disk to check it. But when it says something is wrong, you gotta repair it, or bad things will happen.
(Note: These steps relate to Snow Leopard.)

Steps:

1. Every cautious person should always have a backup handy just in case. There is a chance this can erase some data or just wipe out your entire hard drive. So, keep a backup handy.
   
   
2. We are not going to reinstall Mac OS X.
3. Are you sure you need to repair the disk? You can check this by going into Disk Utility, clicking your disk in the sidebar, and clicking Verify Disk.
4. Save your work in all open apps.
5. Turn on the sound to a point that you can hear it. It will be helpful for booting into Safe Mode after you repair the disk.
6. Insert your install DVD. Don't do anything with it.
7. Restart your Mac (Apple Menu > Restart).
8. When you see the white screen, Start holding down C. Don't let go until you see the usual Apple logo and spinning wheel.
9. Click the right pointing arrow in the dialog box that appears. Don't do anything else unless you want a new language.
10. You are going to repair your first disk with disk utility... I know what you're thinking: Why did we do all that just to get to disk utility? Good question. THIS disk utility is the only one that can repair disks.
11. Click the right pointing arrow in the dialog box that appears. Don't do anything else unless you want a new language.
12. Go to Utilities > Disk Utility. (Utilities is in the menu bar.)
13. Select your disk in the sidebar and Click Repair Disk. Wait. Don't turn off the mac.
14. You repaired the disk. Now all we have to do (Well, in the installer, at least) is to get out of here.
15. Quit Disk Utility and choose Utilities > Start-Up Disk.
16. Click the disk with Mac OS X and click Restart.
17. Hold the shift key as soon as you hear the start-up sound. Don't press it before the sound.
18. Keep holding it until a loading bar appears at the bottom of the screen. Booting will take much longer than usual, so just wait.
19. You are in Safe Mode. The sound doesn't work, you can't use the internet, and the dock lags.
20. Log in and launch Disk Utility.
21. Click your disk in the sidebar and click Repair Disk Permissions.
22. When that is done, normally restart.
23. When your mac boots normally, restore any lost data and get back to what you were doing.
    Source: www.wikihow.com