Thanks for the replies everybody.
I have cleared all my cookies in both IE and Firefox and am still getting the problem.
Unfortunately I know nothing about sessions... I think that maybe I am about to learn!
By forum host do you mean my hosting company? How should the sessions be handled?
A very frustrating problem which I fear will become more so if the problem is with my hosting company as I have no confidence in their customer support whatsoever! Maybe a change of company is called for!
Thanks again everybody!