Collect the Intel Pieces (enemy laptop computers) hidden throughout the game to unlock the cheat options. The number of pieces you have determines which specific cheats are available.

CoD Noir: 2 Intel Pieces. Allows black and white graphics.
Photo-Negative: 4 Intel Pieces. Inverts the colors.
Super Contrast: 6 Intel Pieces. Increases the contrast.
Ragtime Warfare: 8 Intel Pieces. Silent movie mode.
Cluster Bombs: 10 Intel Pieces. When a frag grenade explodes, four more will explode in a cross-shaped pattern.
A Bad Year: 15 Intel Pieces. Enemies explode into tires when shot.
Slow-Mo Ability: 20 Intel Pieces. The game slows to half speed when you melee.
Infinite Ammo: 30 Intel Pieces. Reloads are no longer necessary. This has no effect with single-shot weapons such as RPGs, C4, or Claymores.