To automate the sign-in process, check the box in the sign-in window next to “remember my password”. This illuminates a further option: “sign in automatically”. Check this box, and there you go. We don’t recommend this setting if anyone else uses the same computer… ;o)
To disable automatic sign-in, click the down-arrow in the upper-right corner of the Messenger window to open the Main menu and click Sign Out. In the sign-in window that opens, un-check “sign in automatically”.