Try right-clicking the Extension Manager.exe and then selecting Run as Administrator. It should then be able to install the extension. Unfortunately, EM doesn't ask for elevated privileges when installing an extension into a Program Files directory, which is pretty unbelieveable, given that Vista launched almost 5 years ago. Or you can just turn User Account Control off.
Thanks for pointing me in the right direction John. Right clicking on EM.exe was the solution, but what worked was not running the programme as an administrator but changing the compatibility mode to windows XP. The extensions installed fine after that,
Thanks again, Neil