I noticed that as well, seems it opens the window outside the screen.
To resolve open the task manager by right-clicking the taskbar and in the applications tab, find "Windows Live Messenger" and right-click on it and select Maximize. Then close the contact list and open it again.
I think that should fix it...
Window Manager is discontinued, for WLM try the new, Enhancer for contact list docking, auto-hide, hide taskbar button, remove button flashing and remember keyboard layout in conversation windows
Messenger Plus! Live Enhancer
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