Using my Macbook for one and half year now, crash it several times (cannot count anymore), installed countless of fresh OS X 10.5.

Okay, now hope my post below will save your ass and precious time.

Your Mac OS X Issues

If you have problem like below:

• Cannot start up Mac OS X Leopard, freeze on start up
• Never get into login screen, or stop in the gray Apple screen
• After booting, go to gray screen, wait a long time, and mac is shut down by itself.

It might be caused by failed update (you tried to update 10.5.2 to 10.5.x) or some system files corrupted, or some devices failed to load.