On the disk operations screen, select play a saved game. When you are asked to insert a saved game disk, leave disk two in and press the left mouse button. When the list of saved games appears, click on the first slot. You will now be rewarded with an extra hard opponent.
During play hit 'RETURN' and then type in any of the following and hit 'RETURN' once again.
"ICEMAN" -- Freezes Asteroids (repeat to unfreeze)
"LOADSADOSH" ------------------------ For $100,000
"SKYSCRAPER ------------------- "Instant buildings
"WIDGET" ---------------------- To Gain Blueprints