Restart while holding down the shift key to start up in safe boot mode. When the progress bar shows up you can let up on the key.
This forces a directory check of the startup volume and loads only required kernel extensions (some of the items in /System/Library/Extensions). Also disables all startup items and login items. This can help resolve software or directory issues that may exist on the startup volume.
Disconnect all peripherals from your computer.
Boot from your install disc & run Repair Disk from the utility menu. To use the Install Mac OS X disc, insert the disc, and restart your computer while holding down the C key as it starts up.
Select your language.
Once on the desktop, select Utility in the menu bar.
Select Disk Utility.
Select the disk or volume in the list of disks and volumes, and then click First Aid.
Click Repair Disk.
Restart your computer when done.
Repair permissions after you reach the desktop-http://docs.info.apple.com/article.html?artnum=25751 and restart your computer.
You are still under warranty. Either call Apple Care or
take the computer to an Apple store or an AASP. Whichever is more convenient for you.
Any other ideas?
Mac OS X v10.4–10.6
Mac OS X v10.7
(Btw, if you need help, it's a good idea to specify exactly what Mac OS X version you are using. Advice for one version may not be applicable to a later or earlier version.)