Serious Sam II - Platform: XBox

To unlock the cheats for the XBox version: 
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Go to the main menu

While holding the right trigger, press 
White + Black + White + White + Black + White + Black + White

The cheats menu can be accessed by pressing F1

Cheats:
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At the main menu, hold R and press Black, White, Black, White, Black, 
White repeatedly until the cheat option appears at the bottom. Then, 
press X when at the menu to access the cheat menu. The cheat options 
can also be activated in online games, but may require the game host 
to first enable cheat mode.

Armor in Lava stage:
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Just before you get the mini-gun, there is a door on your right. 
Open it and there will be armor on the other side. You must to step 
on the correct letters; the order is: S, E, R, I, O, U, S.

Duke Nukem Forever reference:
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There are several passing remarks in the game about a "Blonde guy taking 
forever". This a reference to the as-yet-to-be-published PC game 
Duke Nukem Forever.

Defeat Zum-Zum:
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In the level when you must defeat Zum-Zum, you must first destroy all 
of the giant sunflowers so that Zum-Zum cannot regain its life. 
Once you destroy the sunflowers, you can attack Zum-Zum and kill it 
before it recharges.