Just tried creating a new user account, and you're quite correct!When I run it in this account, Media Portal has no difficulty in connecting the internet and downloading the latest weather. Bizarre indeed.
I don't really want to delete and recreate my main account, as the system is pretty darn stable at the mo.
Looks like Internet Explorer 7 Beta is the problem in some way. I uninstalled it from my htpc, and hey presto - MyWeather works again. It must have something to do with the order in which MediaPortal and IE7 Beta are installed. If MP is installed after IE, it works ok (like on my laptop), otherwise the errors occur...