I have the exact same issue as netza, who originally started this thread. After trying various printscreen utilities like Gadwin, Screenshoter and ZScreen, I found that the issue was indeed with Windows 7. I also found that changing the Bioshock 2 in-game Graphics Option to "Windowed Mode" does result in being able to take screenshots, but playing that way takes away from the game play experience, and results in screenshots that have to be edited to remove the top "window chrome."
Computers are supposed to make things easier, not more difficult. Does anyone have a solution for this other than the workaround I discovered?