Accessing Cheats
You can access cheats in the single-player campaign in the "Options" menu found on the pause screen after you beat the game once. Once you collect enough intel for the listed cheat, you can enter this menu to activate the cheat. Any time you die, the effect goes away so re-enter the menu to turn it back on.
Intel 2-6
The first cheat you will earn changes the look of the game. Collect two pieces of intel to play in the "Noir" mode that gives the graphics a gritty black-and-white feel. Find four pieces of intel to play in "Photo-Negative" mode, which inverts the color. Give the game a "Super Contrast" look after collecting six laptops.
Intel 8-15
Use paintballs as your weapons by collecting eight pieces of intel. Get access to cluster bombs after notching 10 pieces of intel. This will cause thrown grenades to explode and then create four more grenades that explode in a larger radius. The odd "A Bad Year" cheat you earn after finding 15 pieces of intel makes dead enemies turn into tires.
Intel 20 and 30
Collect 20 laptops to turn on slow motion. When you press the melee button, the action will move at half the normal speed. Find 30 pieces of intel to get infinite ammunition. Not only will you have unlimited ammo with this cheat, but you also do not have to reload. This does not affect equipment like claymores and C4.