Enter the following cheats at the Extras option in the Main menu.
1980 skin:
NAILGUN
1990 skin:
WOODCHIPPER
2000 skin:
CEILINGFAN
Unlock Everything:
Enter V PIRATE as a profile when you start a new game.
Weapons Unlocks
.40 cal Submachine Gun:
Beat Gnucci Estate Level
.45 calMachine Pistol:
Beat Chop Shop Level

.50 cal Semi-automatic Pistol:
Beat Igor Baltiysky Level
5.56mm Machine Pistol:
Beat Grey's Funeral Home Level

5.56mm Assault Rifle:
Beat Central Zoo Level
5.7mm Submachine Gun:
Beat Fisk Industries Level

7.62mm Assault Rifle:
Beat Pier 74 Level

Auto Shotgun:
Beat Takagi Building Level

Battle Rifle:
Beat Stark Towers Level
Hand Cannon:
Beat Gnucci Level

Machine Gun:
Beat Grey's Funeral Home Level

Revolver:
Beat Lucky's Bar Level

Sniper Rifle:
Beat Grand Nixon Island Level

Flame Thrower:
Beat Igor Baltiysky Level
Grenade Launcher:
Beat Pier 74 Level

Anti-tank Weapon:
Beat Pier 74 Revisited level