Adobe/Macromedia created the AC_RunActiveContent.js to resolve a Flash Player issue with Internet Explorer. I'm not certain if Microsoft has resolved the issue on their end, but I still believe that having this file available is necessary.
I think you should be able to find the file locally on your PC with a simple search... But try looking at the following location first;
Windows XP / Dreamweaver CS3
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C:\Documents and Settings\<your user name>\Application Data\Adobe\Dreamweaver 9\Configuration\ActiveContent\IncludeFiles
I hope that helps.