Help! Can't login after software update!

Question

I went to update iTunes today, the latest security update, and iPhoto update. When the restarted, iTunes kept crashing which it has never done before. So I restarted the iMac and now it has been stuck at the point of the login window but with some really abnormal behavior. The login window appears for a half second then disappears.
I tried the command option p r combo and that didn't work. Please help me with this issue!

Answer 1

Have you Archived & Installed yet? The issue is that your operating system is unable to correctly launch any applications; the login window itself is an application located in /System/Library/CoreServices.
This could have been caused by many factors, but most likely the Security Update didn't install properly, leading to a nonfunctioning system. An Archive & Install will unpack a fresh system folder, replacing the broken files, while leaving your user data intact.
Naturally, if the underlying issue here is with your hard disk or another hardware component, an Archive & Install will not help, and I would recommend repair if the reinstall fails to correct your problem. As always, be aware that if you don't have a current backup of the data on your computer, you could stand to lose information permanently as a result of the Archive & Install.
If you're comfortable doing so, perform the Archive & Install. It will likely yield you a working computer. Steps follow:
First: Insert the Mac OS X 10.5(.x) installation DVD. Reboot the computer.
Second: Once you see the screen turn black, but before you hear the boot chime, press and hold the alt/option key until you see the bootpicker (a row of icons representing available bootable volumes).
Third: You should see your hard drive listed ("Macintosh HD" by default) as well as the Leopard DVD. Select the Leopard DVD with your mouse or the arrow keys and press Return or click the arrow. The computer should start booting from the DVD.
Fourth: Choose your language, then agree to the license terms. Select your internal hard drive from the list of available volumes, then click the Options button.
Fifth: Ensure the middle option, "Archive & Install," is selected with the checkbox below checked. This checkbox preserves your user information and network settings by re-integrating them into the system when the Install is finished. This means that you will start up to the same list of users with the same data inside their home folders. If all goes correctly, this means you will not lose any data associated with your user accounts.
Sixth: Move on and click Customize if you want to remove languages or printer drivers. For the Apple-recommended experience, leave it all checked. Proceed to the install.
When finished, you should be sitting pretty. Remember, data loss is always a possibility. Always keep a recent backup for use in these emergencies.
I wish the best of luck to you!