Invisible/Next level:

During the game press left shift and f11 at
the same time. You will now get a cheat
menu. Type HOLLOW MAN (enter) to be invincible.
Hit enter to get back to the game. 

To win the level and get to the next level, you
go to the cheat menu (left shift and F11) and 
you type CLINT (enter). To get back to the game
hit enter.