Go to your 'MyDocuments' folder on XP (vista/Win7 guys, whatever that location is called on yours) and find either your EAGames directory or BFBC2Beta  (that's what it was on mine) directory.
Open your 'settings.ini' file in notepad and change Fullscreen=true to Fullscreen=false

Your game will now be running in windowed mode, adjust your ingame resolution to fill the screen to your hearts content.

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with windows vista and windows 7 i believe pressing alt + Return(enter) this will switch between full screen and windowed mode

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