Ryanair no longer operates to Paris Beauvais from any UK airport, except Glasgow. So, unless you're travelling from there (or from a non-UK airport, such as Dublin) your question is rather academic anyway ;-)
Getting from Beauvais to Versailles by public transport is fairly easy. The bus service takes 1 hour 15 minutes to reach Porte Maillot. The fare is �13 each way. You can check the times of the buses here:
http://www.aeroportbeauvais.com/horaire_bus.ph p?lang=eng
Porte Maillot is on RER Line C. Southbound trains run towards Versailles - Rive Gauche station (at the end of Line C), but you'd probably need to change trains at Champ de Mars Tour Eiffel. (RER services are swift and frequent).
These car hire companies operate at Beauvais:
http://www.aeroportbeauvais.com/location_voi.p hp?lang=eng
ViaMichelin gives the distance from Beauvais to Versaille as 95km, with the journey taking about 1� hours, but you'd almost certainly encounter some heavy traffic along the route.
http://www.viamichelin.co.uk/viamichelin/gbr/t pl/hme/MaHomePage.htm
Chris