Cheat:
 Go to RUN on your start menu. Type in - HL.EXE-CONSOLE 
 hit enter. 
 You will have to start the game like this everytime you 
 want to cheat. The only thing different will be a console 
 button in the menu screen. 
 Click on it then type in - SV_CHEATS 1 - this will 
 enable cheating. 
 hit ESC to go back to the menu screen. Load you game now. 
 When your game is started stop in a safe place and hit the 
 Tindle key ~ on your keyboard. (top left under Esc button)  
 When the screen pops down keyin - SV_CHEATS 1 - enter. 
 Then keyin /god for god mode or /noclip to fly through walls 
 and stuff or keyin /give_ammo buckshot or replace buckshot 
 with the following if you want other stuf - mp5clip,
 gaussclip,egonclip (egonclip smetimes crashes the game)