It would have helped to know what problems you are experiencing with BT Mail. Furthermore, it would have been easier to explain how to empty your cache if you had indicated the Browser to which you are referring.
Anyway, have a read of the following and, if necessary, post again. There is bound to be someone in AB who can 'come-up' with an answer to your problem.
Nothing wrong with having an old lap top or having it loaded with Vista. At the moment I am posting from my HP Desktop and its operating system is Vista.
I prefer Firefox to Internet Explorer, although some Techies swear by Internet Explorer and I know of others who consider it useless.
By all means try clearing your Cache it may help. :-