Question: If you sign up for yahoo messenger , how do you keep it from popping up when you are not the one on the comput 

Answer: I think I know what you mean... If you dont want Yahoo Messenger to open automatically when your system starts. Then you must go to the menu... Messenger > Preferences > Select the General Tab & then untick the "Start Yahoo Automatically" option.
This question from the Yahoo Q&A forum is categorized in the topic: Computers & Internet