Cheatmode:

Codes are unlocked by performing certain tasks during the game. A message with the code 
will appear to confirm successful completion of the task. Codes cannot be enabled until 
they are first unlocked in this manner.

[Note: Use the following trick keep cheat codes enabled with a new game. After completing 
the game and time trials to unlock the use of the cheat codes, load a level and enable 
the "One shot kill" and "Bulletproof Lara" cheats. Quit the game and start a new game. 
The cheats should remain active. Note: This does not work with the weapon codes.]