Besides the race management software you will need something to decode the transponders as they cross the finish line, you said there is cableing already in place. If it is the loop that is there you may need a AMBrc decoder box as well as the cableing to connect the loop to the decoder box and to the laptop. I beleive the decoder box is connected to a pc via a standard serial cable.Something like this would get you close:
http://www.amb-it.com/ambrc-base-system-c-16_41.html