Make back up copies of all your original files you edit.
First you need to activate the console.
Edit the config. File using notepad located in C:\Documents and Settings\your name\My Documents\US2\.. and type in the following text :
bindc +DIK_GRAVE (console_showhide)
Under this existing command line:
bind +DIK_G (change_grenade)
Save the changes, start the game and press [~] to open the console:
type in sv_cheats 1 to Enable cheats – it willconfirm the value of SV_cheats set to 1
then type in game_godmode 1 to activate god mode
Note: Do not edit the config file in game installed system folder by mistake!
(supplied by: davidlake)