A Windows Update is available to users who are located in member countries of the European Union allowing for choosing a different standard browser for the system. The update is available for download through Windows Update.
Read more here: http://techblog.avira.com/2010/03/08/the-browser-choice/en/
And other infos are here:
http://microsoftontheissues.com/cs/blogs/mscorp/archive/2010/02/19/the-browser-choice-screen-for-europe-what-to-expect-when-to-expect-it.aspx