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If you use windowed mode you can be playing EU2 and speaking in ICQ at the same moment without tabs and without the fear of staying behind in time in EU2.
If ALT-tabbing out of the game disturbs the multiplayergame - then how do you use for example ICQ? To access it and type a message there you would need to get EU2 in the background or is there some other way?
Ampoliros and BurningEGO already answered this question, but I like to add that this is quite an old thread and you should take some information in it with a heavy grain of salt.