Ah ha, here we go.
Thanks for the input guys.
I've been using my account details for various other Yahoo groups and didn't have a problem, then all of a sudden they just wouldn't work.
I have no idea if it had anything to do with it but I've just formatted my hard drive, re-installed XP, etc, etc, and here I am again. The reply buttons are working for me now. I format my hard drive every so often and re-install everything, it was due for it again. All sorts of crap can gradually build up over time and 2 days of re-installing everything can beat many days of searching for answers to operating system problems.
Keith.