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I stumbled across this info when looking for a Javascript-debugger similar to Firebug for IE.

MSE7 is nothing short of a full blown script debugger in an Visual Studio outfit, it ships with Office 2003 SP3 : Filename found here.
How to enable the debugger :
Uncheck the options:

Tools->Internet Options…->Advanced->Disable Script Debugging (Internet Explorer)
Tools->Internet Options…->Advanced->Disable Script Debugging (Other)

The debugger itself looks like this: