You're welcome. I don't use Outlook, but in my newsreaders they have a link to their respective support forums and I would get that "file not found" Windows dialog. Every time I reinstall Firefox it comes back. I use this .reg script to get rid of it:
Windows Registry Editor Version 5.00
All it does is delete those ddeexec keys. I wasn't sure if it was the same fix because there's another mozillazine article that I found originally, but it seems related. Anyway, if you reinstall FF and stuff happens, that reg script might fix it.