Unlock all levels
Press [Left Control]+[Left Shift]+[F9] while in the main menu, then click on the 'Play IGI2' option, and all missions will be unlocked.
It is important to stay hidden during the fights, so don't stay long in one place! Try to aim and shoot from long distances, and after every shot get in the cover and wait for a while.
Related: IGI 2 Covert Strike Review