Shoutbox

Updating Windows will NOT cause Messenger to open your mailbox in any browser other than IE. This is because, while links are passed to the default browser (just like opening an HTML file on your drive), Messenger uses IE directly to launch the mailbox i.e. you cannot change that behaviour without hacking the Messenger executable (highly dangerous and illegal) or hooking (difficult, but possible: take Plus! as an example).

EDIT: My XP PC is fully up-to-date. SP1 is just bugfixes and security updates, and while IE is a Windows compnent, Messenger is not, and is therefore not updated by Windows Update.