Thanks to all for your advice and help.
I uninstalled CFS3, OFF, Firepower and the EOT add-ons. I also deleted all related items in registry. I also deleted all related folders and subfolders in Explorer.
I reinstalled CFS3 3.1, flew a couple of training missions, and then reinstalled OFF.
All of the options on the start screen were accessible and all opened in full screen except 'Workshop'.
I created pilot, flew a mission for almost a minute before meeting an inglorious demise some miles from the end of the runway.
Instead of displaying in full screen mode 'Workshop' will only display in a small splashscreen, leaving some options out of view, including the essential 'Main Menu' escape.
So, I opened MS Task Manager and closed down 'Workshop'. Then I rebooted my PC and lowered the display options to 800x600 for Windows Desktop, CFS3 and OFF.
I was then able to see all of the options in 'Workshop' including the 'Main Menu' escape.
I reset the aspect ration to 16:10 and exited to Main Menu before exiting OFF.
I then rebooted my PC, and reset screen resolution for Desktop, CFS3 and OFF to 1680x1050, rechecked CFS3 and opened up OFF again.
Everything else seems ok but 'Workshop' still opens in a small screen and is therefore unusable.
I think the 'Workshop' is designed to open in full screen like the other start page options but for some inexplicable reason it will only open in a small splash screen on my PC. Maybe some little registry peculiarity?
End result so far is that the OFF Sim is playable but only with the default settings.
Seems unlikely this will be resolved
Thanks again for all your advice.
(My PC is custom built with Windows Vista Ultimate (x64) Service Pack 2, ASUSTeK Computer INC. P5QL Rev X.0x Mainboard, 2.67 gigahertz Intel Core2 Quad Q9400, 8.0G Ram, and NVIDIA GeForce 9800 GT).