We use Zround, really great and easy to use.
It has the ability to run dual screens, so we put one on a lcd screen infront of us and the 'real' one run on the laptop. The start is lights and sound!
Try it, then just use an extra monitor turned at the drivers so they can see...
Best to use a cheap projector, we are searching for one!
It's spanish website, but run the site through google translate.
The install languge is spanish, english, swedish and some more.
Great program that is free, works with most hardware, We use the AMB system on a rclapconter hardware. Lots of features and even saves best laptimes.
Also have graphs about who overtook who from start to finish, can choose staggered start or standing start, timed race or laps race...