Can't claim any great knowledge of OE6 but have you tried renaming the file that contains the messages and creating a new empty data file? I can't remember where Windows stores them by default - but I do remember it's kinda obscure! The MS Knowledge Base has the details somewhere.
If that does the trick I would assume that the data file is corrupted in some way. I'm sure you could import the messages you want into the new file from the old one - maybe even easily!
If that doesn't fix it there might be a problem in the Registry. Regclean might fix it - I don't think MS offer it anymore but you should be able to fin a copy on the web without difficulty. And I don't think they recommended it for XP, but I've used it without problems on XP systems.
Hope this helps.