The problem comes from a poorly written Logitech driver that in turn installs a program called the Application Enhancer (aka APE) into the System folder. APE works fine in Tiger, but not in Leopard, hence the blue screen.

Fortunately, there are several fixes and you don’t have to lose any of your data!

Here is the solution I used to fix the problem:
(1) Start up in single-user mode by holding Command-S after restarting the computer. Note that single-user mode always uses the US English keyboard layout.

(2) Type these commands, each on a single line followed by Return:

/sbin/fsck -fy
/sbin/mount -uw /
rm -rf /System/Library/SystemConfiguration/ApplicationEnhancer.bundle
reboot

Check out these resources for more information on the subject:
Apple Knowledge Base Article 306857
Mac OS X 10.5: “Blue screen” appears after installing Leopard and restarting
http://docs.info.apple.com/article.html?artnum=306857

Daring Fireball’s (John Gruber’s) post on the subject, which is how I found the Apple KB article in the first place:
http://daringfireball.net/2007/10/blue_in_the_face

I notice your post is dated 1/19/2008 and the Apple KB is dated 11/08/2007. I hope you haven’t reformated that computer…

Good luck everyone!

/eh

*hugs*

I’m really sorry, dude… on the one hand, you did make some mistakes, but it really shouldn’t have been that catastrophic of an experience. You would think (hope?) that there would be some kind of warning when an install is going to completely fuck your shit up. Sad.

I am sorry for your loss.