Serious Sam HD Cheats :
Code - Cheat Menu
Press ~ (or the key above Tab) to open the console. Type cht_bEnableCheats=1. Press ~ , Esc, F1 to open the Cheat Menu (Max Health, All Guns, Level Select etc.) At the bottom of the Menu the Cheats option will now appear. Go to this and type cht_bEnableCheats=2 for more options like God Mode and invisibility.