One way to fix a Mac that won't login

So the other night, while covering Sundance Film Festival for Blogging Sundance and Cinematical, we had a problem with Jason Calacanis's Powerbook G4. It would boot and run through the loading Mac OS X screen very quickly, but at the point where it was supposed to load the login screen, we were greeted by the default blue OS X background and the mouse pointer arrow and nothing else. And it just stayed there.

We tried rebooting, we tried zapping the PRAM, and we both booted into single user mode and booted the computer in target disk mode, attached to my laptop, and performed a disk check on the computer. No problems with the disk. The machine just wasn't booting.

After throwing up my hands and proclaiming there to be some odd hardware failure, it suddenly hit me: maybe the login window preferences were borked. We booted into single user mode again (reboot and hold down the command s button). I ran all the commands that you have to run in order to write to the disk (/sbin/fsck -y followed by /sbin/mount -wu /). Then I ran:

cd Library
cd Preferences
mv com.apple.loginwindow.plist BACKUPcomappleloginwindowplist

This makes a backup copy of the preference file associated with the login window and erases the original file in the process. After I was done, I typed reboot at the prompt and hit the return key. OS X rebooted and was forced to create a new preference file for the login window, thus fixing the problem. Huzzah!