Apple says to Archive and Install Leopard
According to the document, some users found that after completing an upgrade installation of Leopard and restarting the computer, a "blue screen" appeared for an "extended period of time." The company blamed "third-party 'enhancement' software installed that does not work with Mac OS X 10.5 Leopard."
The primary fix is to reinstall Leopard with the Archive and Install option.
Apple also offered a terminal process:
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 these commands, each on a single line. Important: Type each command carefully, misuse of the rm command may damage other files. rm -rf /Library/PreferencePanes/Application\ Enhancer.prefpane rm -rf /Library/Frameworks/ApplicationEnhancer.framework rm -rf /System/Library/SystemConfiguration/ApplicationEnhancer.bundle rm -rf /Library/Preferences/com.unsanity.ape.plist Restart normally. If the issue persists, use solution 1 above instead.
By my reckoning, that amounts to a reccomendation by Apple (and I'm sure they would disagree). Now, Archive and Install is the recommended choice of many longtime Mac professionals, including the Apple Core's own Jason O'Grady.
Here are some of our recent Apple Core articles about things to consider before you install Leopard:
Mac OS X Leopard installation as a spiritual practice
Leopard pounces, don’t get mauled