If you are as lazy in activating Windows, as I am, you might also run into this problem:

When Longhorn hits the activation grace period, it will not log you in, unless you activate. To do this over the internet Vista will use your Internet Explorer proxy settings. So far so good, but what if that happens on the road with different proxy requirements?

Windows 2003 and XP would let you manually set the right proxy if it failed on the first attempt. Not so Vista: you cannot change the proxy settings in the activation dialog.

Here is the solution: Click on "Buy a new License Key Online" and IE will try to connect (and fail). You can however go into "Tools -> Internet Options" and correct your proxy settings.