After google-ing it tonight, its pretty complicated, but as is my understanding they use the horizontal and vertical lines of a crt screen, and also something to do with the white flash you get when you press the trigger- this highlights the intended target and the light sensor in the gun works out where you shoot. or something
...basically, old light guns dont work on any LCD scrren due to them being pixels rather than lines.
ok googled it, thats a bit of a mind humper. i suppose you could simulate the line drawing etc on a monitor, maybe someone has written the software that person would be very sad indeed...