Locate the file'autoexec' in your game directory and make a backup of it. Then open it using a text editor and change the value of the 'PlayerClip' line from '1' to '0'. When you now play the game you will be able walk through walls.
If you want to play the game without worrying about running out of ammunition then press 9 during gameplay and type 'mpammo' (without the quotes) and then press 'Enter'. If the code has been entered correctly you will hear a confirmation sound.
To enable this cheat find the 'autoexec.config' file in the game directory and create a backup copy of it before proceeding. Then using a text editor change the value of the 'PlayerTakeDamage' line from '1' to '0'. Note: the spotter is not invincible and can still be killed.