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.
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