We are traveling from our hotel in Wandsworth to Windsor Castle. It looks like the best route is Southwest Trains going from Wandsworth Town to Weybridge to Windsor Eaton Riverside. We will be purchasing a 7 day Transit Card on the Oyster Card. Will the train fare just be from the point we leave zone 2 and then automatically go to the Oyster card ?
Also check the route from Paddington to Windsor & Eton Central - it's much faster than the train from Waterloo, possibly even if you are travelling from Wandsworth.
You cannot use Oyster out as far as Windsor at all, you need to buy a specific ticket. If you have a pass as far as Zone 2 it is worth going to a staffed ticket office and asking for a ticket from the edge of Zone 2 to Windsor - you can't do this on a ticket machine.
Another advantage that Windsor and Eton Central has is that it is directly across the street from the entrance to Windsor Castle.
If you arrive at Riverside it is a bit further, and you have to walk up the hill around the outside of the Castle wall up from a much lower level.