Mac OS X 10.6: If you see a blue screen at startup
If you see a blank blue screen or a blue screen with a colored pinwheel when you try to start your computer, but starting up doesn’t progress, you may have a problem with damaged or incompatible startup items or software.
Note:To print these instructions, open the Help Viewer’s Action menu (looks like a gear) and choose Print.
Try the following:
If you are starting up from a DVD, make sure it is the disc that came with your computer. Discs from other Mac models may not work with your computer.
Restart the computer in safe mode. If the computer successfully starts up in safe mode, choose Apple menu > Restart to see whether the computer will restart normally.
If you see the login window prompting you for a user name and password, but starting up doesn't progress, you may have incompatible login items.
Repair your startup disk or disk permissions by starting up from your Mac OS X installation disc and using Disk Utility.
Back up your disk and then reinstall Mac OS X by starting up your computer with your Mac OS X installation disc.