Leopard: How to Avoid the

Several users have faced a problem leading to a "blue screen" to be displayed during the first boot following Leopard installation.
Apple published a TechNote on this subject. The problem seems to come from "enhancement" software incompatibility with Leopard (such as SwtchRes).

Solution 1: Reinstall Leopard (recommended)
It may be necessary to perform an Archive and Install installation of Leopard. Archive and Install moves your existing Mac OS X system files to a folder named Previous System, and then installs a new copy of Mac OS X on the selected volume. Mac OS X–installed applications, such as Address Book and Safari, are archived, and new versions are installed in the Applications folder. Applications, plug-ins, and other software may have to be reinstalled after an "Archive and Install." This is covered on page 7 of the Install & Setup Guide included on the Leopard DVD.
You will probably want to check "Preserve user and network settings" when starting the installation.
Note: After installation, verify each third-party software product is compatible with Leopard before reinstalling it, especially any application "enhancement" software.
Solution 2: Use the command line (advanced) to remove application enhancement software
Try this solution if you are comfortable using Terminal and have certain application enhancement installed. The software may be removed following the below steps:
Start up in single-user mode by holding Command-S after restarting the computer.
Execute these commands, each on a single line:
/sbin/fsck -fy /
/sbin/mount -uw /
Execute this command on a single line. Important: Type the command carefully, misuse of rm may damage other files.
rm -rf /System/Library/SystemConfiguration/ApplicationEnhancer.bundle
Restart normally. If the issue persists, use solution 1 above instead.