Marie,
Have you tried starting up in the Safe Boot Mode? (It is similar to starting up with the extensions off in Classic.)
To start up into Safe Boot Mode do this:
1. Be sure the computer is shut down.
2. Press the power button.
3. Immediately after you hear the startup tone, press and hold the Shift key.
Note: The Shift key should be held as soon as possible after the startup tone but not before.
4. Release the Shift key when you see the screen the gray Apple and progress indicator (looks like a spinning gear).
During the startup, you will see "Safe Boot" on the Mac OS X startup screen. To leave Safe Mode, restart the computer normally, without holding any keys during startup.
From Safe Boot perhaps you can run Disk Utilities and or fsck –y.
Once you get up and running again, you may also want to ensure Energy Saver is not active and that you have not created a hot spot that automatically puts the machine to sleep.
See Apple Knowledge Base Documents 107392 and 107393 for details on Safe Boot.
HTH
Harv
