alpha_lima_xray,
Don't pay a taxi too expensive. Heathrow has connections to the Piccadilly Line so the underground connections are good.
Do this call up London Travel Information on 0207 222 1234 they would give you the easiest route and the cost to get to Windsor (make sure to tell them u don't have an oyster card) . Or go to
English - Journey Planner - Transport for London. Theres a train that goes from Vauxhall all the way to Windsor & Eton. You could get on somewhere in between.
And all this should cost you around £10 return, depending on where you are. And you can give me the remaining £30 if you want
Good luck for the exam.
(I'm going too.)