When the "Press Start" screen appears, hold Select and press Square, Triangle, Circle, Triangle, Triangle, Square, Circle. You should hear an asteroid exploding. If not, try again. Now, anytime during the game press Select and Start at the same time. A small screen should come up asking you what level and zone you want. You will also get the option of turning off collisions. To activate the cheat codes, press R1, R2, L1, and L2 at the same time.

At the title screen press Square, Triangle, Circle, Triangle, Triangle, Square, Circle. To activate level select, press Start + Select while playing, then press L1 repeatedly until the desired level appears.

Press pause, then enter the following:

99 Lives: Up, X, Down, Triangle, Left, Square, Right, Circle

Invincibility: Down, Down, Up, Up, Circle, Square, Triangle, Triangle

Beat the game at expert or hardcore

At the title screen press and hold SELECT, then press Triangle, Circle, Circle, Triangle, Square, Circle, Square.

At the Title screen, hold Select and press: Circle, Circle, Circle, Triangle, Square, Square, Circle.

To get the original game you must go all the way through the first mission until you get to level 15.  Once you get to level 15, continue to play but look for a greenish wire-frame asteroid to come into the screen.

Shoot the green asteroid and big words in green will appear on the screen letting you know the game is unlocked, but don't stop just yet.  Finish level 15 and save your game so you can save the original version for fun. 

To get armegeddon without getting the power up simply hold the fire button for 5 seconds then release. This can be done in 2 player mode too.