Press 'P' to pause the game and type "BREAKFASTINBED". Now unpause the game and infinite lives are yours.
The following does not work on all versions:
On the high score table, enter your name as "SPOT" for infinite lives.
If you look at disk two, you'll see some files called LEVEL2.DAT (for example). If you rename THEEND.DAT (or whatever it's called) to LEVEL1.DAT, you will see the end sequence without playing the game!