Quickest way would be by taxi.
Avoid the insanely overpriced black cabs and call Green Tomato Cars. They’re pretty cheap and reliable, they should be able to get you to Windsor in about 15 minutes. If you call them first they’ll quote you a price before you book.
Windsor is outside the TfL zone and cabbies can charge anything by agreement. Come out of T5 and head across to the buses. The 77 is a blue bus. Unfortunately it stops at Slough bus station for an age, but it is cheap.
The fastest way is to book a cab from Windsor. The train is a pain as it is heathrow to Hayes. Hayes to Slough. Slough to Windsor.
Oh and I cycle Windsor to T5 and then Uxbridge to work daily.