Had the internal error on start up problem. Found a lot of solutions on this list but non worked.
I then took a punt on the Fireworks CS4 Panel Prefs.xml' file and renamed it (I wanted to keep it in case it was the culprit).
Happy to say it was. Problem fixed. Just posting this in case someone else can be spared the hours head scratching I did.
This is just to set this question as answered.