If Windows Live Mail comes pre-installed on your computer, it is automatically set as the default email client, but can change as soon as you get a new application that can handle email (they tend to set themselves as the default, either automatically, or by having a checkbox enabled in their installer). This tutorial explains how to change the default mail handler from within Windows Live Mail, which nearly always does the trick; but we'll also show you how to manually pick it through the Control Panel in Windows 8, Windows 7, Windows Vista, and Windows XP.
Click on the Windows Live Mail menu (top left button), and go to Options > Mail:
Make sure that the General tab is selected at the top: the last section, "Default Messaging Programs", shows to "Make Default" buttons - if Windows Live Mail isn't the current default email program, the button will be clickable (see screenshot). Click on the one next to "This application is NOT the default Mail handler": If it is already the default, you'll see "This application is the default Mail handler" with a disabled (grayed out) Make Default button - as happens once you clicked on the button. As test, click on this email link: a new Windows Live Mail message window should open.
Tip: when Windows Live Mail starts without being the current default, it will show a confirmation dialog. Clicking Yes will restore it as the default. Leave the "Check this setting when I start Windows Live Mail" checkbox checked, so that this operation is automatically performed whenever you launch the application.
If it worked, Windows Live Mail is now your default! Otherwise, read on.
Below are instructions for Microsoft's last four operating systems. When you are done, click this test email link: a new email window should open in Windows Live Mail.