As a floow up to my request - I've just picked up the following from another web page.
'I do not have a fix for you. But my Outlook 2000 did that awhile back and I found I could run OL2000 by running...
From the Start, Run box. From there it worked fine. I ran that way for weeks until I had time to find out what the issue was, but when I went back to just using the shortcut, it worked fine again.
That's all I can share.'
Credit to R Profitt.
This actually works - but does it give some indication of where the problem lies?