You have more than one Windows Live ID — for example one in the office and one privately?  You can link both with each other.  If you do that, you can sign in at a Windows Live website like Hotmail once and access information in both accounts.

1. Open http://home.live.com/ in your browser.
2. Sign in with your first Windows Live ID.
3. On the top right, click on your account name.  A drop-down menu appears.
4. In the menu, click the Link other accounts entry.
5. You may be asked to re-enter your Windows Live password.
6. Click the Link an existing Windows Live ID hyperlink.
7. Enter the password of your first Windows Live ID, and the e-mail address as well as the password for the second Windows Live ID, the one you are going to link.
8. Click the Link button.
9. Now on the top right, click your name.  You can access both linked accounts from there now — without additional sign-in.