If you've ever tried to run the Classic and OS X versions of Internet Explorer at the same time, you'll find that it doesn't work. There was some discussion on Omni's OS X Talk mailing list the other day about this problem, and the answer came down to the fact that both IE's use the same creator code. So when you launch one version, the OS then prevents you from launching the other. Zachery Bir came up with a quick command-line fix for the problem (assuming you have the Dev Tools installed):
% cd /Applications (Mac OS 9)/Internet Explorer 5/Of course, you could also use one of the GUI tools to change the creator code on IE5/Classic. You simply have to change it to something that isn't in use by another program; Zachery chose "MIEC" and that seemed to work fine.
% /Developer/Tools/Setfile -c MIEC Internet Explorer
Mac OS X Hints
http://hints.macworld.com/article.php?story=20011215093334775