Well, my problem has been resolved and it wasn't down to Vista or firewall or anti-virus.
After I registered I requested a name change which I thought had been ignored. It hadn't. I just never received the email.
So, the mIRC on my XP machine is registered with the original name and code and, after retrieving my lost registration
, I have now registered the mIRC on my Vista machine with the updated name and code.
Thanx for the help Khaled.