Can you start it in Safe Mode?
To start up into Safe Mode (to Safe Boot), do this:
- Be sure the computer is shut down.
- Press the power button.
- Immediately after you hear the startup tone, hold the Shift key. The Shift key should be held as soon as possible after the startup tone but not before.
- Release the Shift key when you see the gray Apple icon and the progress indicator (looks like a spinning gear).
- During startup in Mac OS X v10.4 or later, you will see "Safe Boot" on the login window, which appears even if you normally log in automatically.
- During startup in Mac OS X v10.2 through v10.3.9, you will see "Safe Boot" on the Mac OS X startup screen.
To leave Safe Mode in any version of Mac OS X, restart the computer normally, without holding any keys during startup.
Info sourced from Apple support forum
You can also be more proactive:
- Try your install CD that came with your Mac, reboot, hold down the letter C after you hear the chime, and when it boots to the installer, pick a language, and then run Disk Utility by finding it in the top menu.
- Instead of holding down C at startup, hold down the letter D and make sure all your hardware's okay.