No log on promt, just blue screen at start up.

If you see a blank blue screen, or a blue screen with a progress indicator (looks like a colored pinwheel), you may have an incompatible login item.

To find out if you have an incompatible login item:

Shut down your computer and wait 30 seconds.

Press the power button.

Immediately after you hear the startup tone, press and hold the Shift key.

Release the Shift key when you see the gray Apple and progress indicator.

Choose Apple menu > System Preferences.

Click Accounts, then click Login Items.

Make a list of the login items--you'll need to remember them later.

Select all of the login items and click Remove.

Choose Apple menu > Restart.

If this solves the problem, open Accounts preferences again and add the login items one at a time, restarting your computer after adding each one, until the symptom occurs again. (You can start up your computer normally when you're adding the login items.) When you've found the incompatible login item, use the procedure above to remove just the problem item.