You may not realize you have APE - Logitech's mouse driver is known to use it, and AudioHijack Pro's Instant hijack is also known to use it.
If you are getting the blue screen and no loginwindow, you can nuke APE without a reinstall with the following steps:
- Boot into single user mode by holding Apple+S on startup
fsck -fy /
/sbin/mount -uw /
rm -rf /Library/Preference Panes/Application Enhancer.prefpane /Library/Frameworks/Application Enhancer.framework /System/Library/SystemConfiguration/Application Enhancer.bundle /Library/Preferences/com.unsanity.ape.plist
sync;shutdown -r now
John Gruber has more background on this in an article he wrote on Daring Fireball here and Apple posted a KB article about this here.