Wow! That's obviously not good.
Questions:
1. Does it get to the logon screen or does it boot to the Darwin shell?
2. Do you have more than one user? Have you tried logging in as another user if you have one?
Can you think of anything that happened just before your computer stopped booting correctly?
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If you're Unix savvy, you might try holding down Apple-S to enter single user mode. This will give you a Darwin shell.
To get you started, it might be worth it to:
cd /Library/StartupItems/
ls
Look in there and see what's there. Also look in /Library/Extensions to see if there are third-party kernel extensions that you could temporarily move out.
**As always, be careful when moving or deleting files **.
Doug