How do I stop this?

[Microsoft Windows]

Since 3 days ago, whenever I turn my computer on, or restart it, I get the Messenger window (see atch) that is first to open. Don't know why, it just started "out of the blue". I have Messenger on my Facebook and my wife's Facebook and both are already open, no log-in needed to use them.What do I need go do to stop this?

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