For all who are using or planning to use the Ad Remover app from www.adremover.org on Windows 8.1: This app is NOT compatible with Flight Simulator 2004 and WILL prevent you from using FS2004 (it will allow FS2004 to start, but WILL CAUSE IT TO CRASH as soon as you attempt to load any flight). The good news is, if you experience this, uninstalling Ad Remover will restore normal operation of FS2004 with no need to reinstall the simulator or remove any add-ons.
I'm unsure as to how this software would affect FS2004 functionality - it runs as a browser plugin. It's possible it may cause problems with the interactive kneeboard but that's about it.
Perhaps it was a coincidence. Perhaps this post it a plug.