Hold down the Shift key at startup.
http://support.apple.com/kb/ts1417Apple support:
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Try a Safe Boot
If you're using Mac OS X 10.2 or later, you can start up your computer in Safe Mode, which includes an automatic disk check and repair. If you're using Mac OS X 10.1.5 or earlier, skip to the next section. A Safe Boot, which starts up your computer into Safe Mode, may allow you to start up your computer successfully using a reduced version of the system software. To do this, follow these steps:
1. Start up in Safe Mode.
2. After the system has fully started up, restart your computer again normally.
If the computer successfully restarts, you do not need to do any more troubleshooting. If the issue persists, try Disk Utility.
If that doesn't work try to startup from the system install disc. Hold "C" at startup.
Try Disk Utility
1. Start from your Mac OS X Install disc: Insert the installation disc, then restart the computer while holding the C key.
2. When your computer finishes starting up from the disc, choose Disk Utility from the Installer menu. (In Mac OS X 10.4 or later, you must select your language first.)
Important: Do not click Continue in the first screen of the Installer. If you do, you must restart from the disc again to access Disk Utility.
3. Click the First Aid tab.
4. Click the disclosure triangle to the left of the hard drive icon to display the names of your hard disk volumes and partitions.
5. Select your Mac OS X volume.
6. Click Repair. Disk Utility checks and repairs the disk