Thanks very much for all of your advice Karleuns,
I tried the complete uninstall in the end and still no joy. I took my machine back to the shop where I bought the card from (Antech in St Ives, Cambs), they laboriously searched for the solution and found it.
Right click on SH3 icon on desktop - select Compatability tab - under Privilege Level heading, select 'Run this program as administrator'.
In the Compatability mode heading I currently have 'Windows XP (Service Pack 2)' also selected.
I now am able to access SH3 once more, unfortunately all of my career info is gone. I now have to start from scratch. But hey, that is a small sacrifice for being able to get back to my beloved U-83

.
Rgds to all