Edit the config.ini file in 

C:\Documents and Settings\MyName\My Documents\US2\..   

and add the following text to the end:

(remember!! no edit config.ini from game installed folder!!!)


bindc +DIK_GRAVE    (console_showhide)

this will allow you to press the ~ key and open the console.

In game, press ~ and write:

sv_cheats 1

and then write:

game_godmode 1  = GOD MODE

(1 -activates, 0 -deactivates)