If you use a plugin like <a href="https://wordpress.org/extend/plugins/private-wp/" rel="ugc">Private WP</a> to make your blog available only to 
registered users, then the auto-configuration ability of Windows Live Writer no longer works for your blog. To make Windows 
Live Writer work again, install this plugin. It will make sure that the configuration files for Windows Live Writer are linked 
from the login page, which does the trick.

WLW Login

Includes the Windows Live Writer manifest link on the login page also, for secured blogs.

dalziel

Using the <a href="http://msdn2.microsoft.com/en-us/library/bb463266.aspx" rel="nofollow ugc">Provider Customization API of Windows Live Writer</a> this adds a link to the head section of your blog, which links to a manifest file. This tells Windows Live Writer to display custom values in the sidepanel for your blog. Currently this will show your favicon if it exists & a watermark image & a link to the comments section in your admin and a link to a stats page set through the options page

Windows Live Writer

Creates a manifiest file using the Provider Customization API of Windows Live Writer to create a custom sidepanel in Windows Live Writer for your blog

WLW Login vs Windows Live Writer