The first game I wrote for the PC!

This game was written during my first year at college while doing my HNC in computing, and ultimately doomed my Amiga to start collecting dust on top of a cupboard somewhere.
It is just yer basic space invaders clone using absolutely NO interrupts. For those of you that don't know (or are too young to remember) what an interrupt is, its an event that INTERRUPTS the execution of the main program loop and allows something else to run apparently at the same time ;-) In the case of this program, its crying out for a keyboard interrupt. Well, it was my first effort - can you do better?