Enable cheats:

What you have to do is beat a level within 
the time given below. When you do that it will
enable that Cheat. 


Level       Cheat              Difficulty     Time

Dam         Paintball Mode     Secret Agent   2:40
Facility    Invincibility      00 Agent       2:05
Runway      DK                 Mode Agent     5:00
Surface 2x  Grenade Launcher   Secret Agent   3:30
Bunker 2x   Rocket Launcher    00 Agent       4:00
Silo        Turbo Mode         Agent          3:00
Frigate     No Radar (Multi)   Secret Agent   4:30
Surface2    Tiny Bond          00 Agent       4:15
Bunker2 2x  Throwing Knives    Agent          1:30
Statue      Fast Animation     Secret Agent   3:15
Archives    Invisibility       00 Agent       1:20
Streets     Enemy Rockets      Agent          1:45
Depot       Slow Animation     Secret Agent   1:30
Train       Silver PP7         00 Agent       5:25
Jungle 2x   Hunting Knives     Agent          3:45
Control     Infinite Ammo      Secret Agent  10:00
Caverns 2x  RC-P90s            00 Agent       9:30
Cradle      Gold PP7           Agent          2:15
Aztec       2x Lasers          Secret Agent   9:00
Egyptian    All Guns           00 Agent       6:00

And the below weapons can be enabled simply by
beating the levels. 


Weapon                    Level

Magnum Antenna Cradle     Janus base
Laser                     Aztec Complex
Golden Gun                Egyptian Temple