If this RC car is one of those simple ones that are controlled just by pressing few buttons (controks are either on or off), then you can use this approach:
Connect a relay card to parallel port. Wire the relay contacts in parallel with the existing button contacts on the device (if those are type make contact when pressed). Now one relay controls each button same way the pressing the same button would. Now with any suitable parallel port control program you cn turn relays on and off simulating the pressign and relesign of those control buttons.. Shoudl work without too much trouble.
More details
http://www.epanorama.net/circuits/parallel_output.html
http://www.epanorama.net/phpBB2/viewtopic.php?t=1636