Stuck in a "loop" at the login window after upgrading to Leopard

Question

I bought a new iMac Oct 26. It contained Tiger, and included a disk for installing Leopard. I easily got the machine up and running using Tiger, but last night I attempted to install Leopard. When the installation was complete, I tried to get it running. A screen asking for my password came up. I keyed that in and hit logon. Back came the screen asking for the password. To make long story short, I tried everything I could last night, and then called Apple Support this morning. Took quite a while, but I was back in business this afternoon. The technician who helped me sent me the email copied below describing the solution. Perhaps it will help someone else? Romelle
Hello from Apple!
Charles at Apple Service & Support thought that you might find this article useful. We hope that it helps resolve your technical issue.
Link: http://www.info.apple.com/kbnum/n306965
Mac OS X 10.5, iMac (Late 2007): Stuck in a "loop" at the login window after upgrading to Leopard
Issue or symptom
After installing Mac OS X 10.5 Leopard on a 20-inch or 24-inch iMac (Late 2007) computer, you may not be able to log in at the login window. Your login name and password are apparently accepted, but after a blue screen appears for a few seconds, the login window reappears instead of your desktop.
Products affected
20-inch or 24-inch iMac (Late 2007) computer with 2.0, 2.4, or 2.8 GHz processors
Mac OS X 10.5 Leopard
Solution
If Leopard is installed
Start up from the Leopard Install DVD and perform an Archive and Install installation (and select Preserve Users and Network Settings).
After the Archive and Install is complete, you should be able to successfully log in. Use Software Update to install any other updates.
Important: Install the iMac Software Update 1.3 for Leopard after the Archive and Install is complete.
If Leopard is not yet installed
If you have not yet installed Leopard on your 20-inch or 24-inch aluminum iMac computer with a 2.0, 2.4, or 2.8 GHz processor running Mac OS X 10.4.x, but previously installed the iMac Software Update 1.2, you should install the iMac Software Update 1.2.1 (Tiger) or later, before installing Leopard.
Thank you,
Apple

Answer 1

If Leopard came pre-installed and this is a new machne, then the disks that came in the box should let you archive install preserving users & network settings. The disks have a grey label, and if you get more than one use disk #1 for this procedure.
Hold down the "option" (also known as "alt" on the new keyboards) key when starting up with disk #1 in the optical drive. Select the install disk icon on the grey screen and click the arrow. Then when you get to the "select destination pane, click the options button after selecting your hard drive as the destination. In options A& I preserving should be checked by default, if not, select them. Click "ok" to dismiss the options pane, then run the installation.
If this does not work for you, call apple support. They want everything to work. Be patient, they are real busy these days.
Slim