Apple has posted the following support doc on their site regarding the reported blue screen problem encountered after updating to Leopard.
Mac OS X 10.5: "Blue screen" appears after installing Leopard and restarting
Issue or symptom
After completing an upgrade installation of Leopard and restarting the computer, a "blue screen" may appear for an extended period of time.
Products affected
Mac OS X 10.5 Leopard
Solution
You may have third-party "enhancement" software installed that does not work with Mac OS X 10.5 Leopard. Use one of these solutions.
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:
1. Start up in single-user mode by holding Command-S after restarting the computer.
2. Execute these commands, each on a single line:
/sbin/fsck -fy /
/sbin/mount -uw /
3. 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
4. Restart normally.
5. If the issue persists, use solution 1 above instead.
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Mac OS X 10.5: "Blue screen" appears after installing Leopard and restarting
Issue or symptom
After completing an upgrade installation of Leopard and restarting the computer, a "blue screen" may appear for an extended period of time.
Products affected
Mac OS X 10.5 Leopard
Solution
You may have third-party "enhancement" software installed that does not work with Mac OS X 10.5 Leopard. Use one of these solutions.
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:
1. Start up in single-user mode by holding Command-S after restarting the computer.
2. Execute these commands, each on a single line:
/sbin/fsck -fy /
/sbin/mount -uw /
3. 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
4. Restart normally.
5. If the issue persists, use solution 1 above instead.
READ MORE