All of the forums use the same login. If you have a Windows Live account, it should work for all of them. Unless you have asked it to retain your username and password, it will ask for them each time you open the browser and go to a forum. I'm assuming you also have cookies enabled for the appropriate site.
Also, some of the forums have been moved to the new format, and some haven't. That might also be a reason you're having to log in twice. I think ASP.Net is in a completely different location from the other forums, too.
RobinS.
GoldMail.com