"newer version available" prompt

I have Windows Messenger disabled but not removed on my XP Home with Norton
AV and firewall. I did not remove it because I understood, perhaps
misunderstood?, that doing so would affect Outlook Express, which I use.
Everything has been working fine for quite awhile, with no icon for
Messenger in the right lower task bar and no appearance of it anywhere.

However, lately I am getting this "newer version available" prompt whenever
I boot up the computer, sometimes right away and always when I start up
Outlook Express. How can I stop this update message from appearing?

Thanks for any help. Fleur