@Graham – nothing unusual at this point. Did you check your user directory for additional "Scripts Panel" folders?
You can find it under:
User / YourUsername / Library / Preferences / Adobe InDesign / Version 5.0 / Scripts
If you cannot detect any Startup Scripts there, I really think, that something went wrong with one of the XHTML scripts that should automatically loading at startup.
What you can do to check is dragging the folders "Export As Xhtml" and "XHTL for Digital Editions" temporarily out of the Scripts folder to the desktop and restart InDesign CS3.
If the error message is gone, I recommend uninstalling and reinstalling InDesign CS3.
If this will fail, there is a cleaner tool at:
http://helpx.adobe.com/creative-suite/kb/cs5-cleaner-tool-installation -problems.html
I personally never needed to use it. Might be of help, if you want to get rid of (nearly) every trace of the Creative Suite on your machine. That said, did you install also newer versions of CS? If so, see the cleaner tool as last resort. Best read the comments on it twice before using it…
Uwe