Trains to windsor from london

Take the train from London Paddington, it’s a 35 minute train ride. You will change trains is Slough which is simple, just change platforms. When you get to Windsor, exit the train turn left, walk up to the street and the castle is across the street.

Both train stations are very close to the castle and you can see them on Google Maps. Here's the train summary from Rick's London 2018 (p. 477), in case you'd rather depart from London Waterloo:

By Train: Windsor has two train stations--Windsor & Eton Central and Windsor & Eton Riverside. London’s Paddington Station connects with Windsor & Eton Central (2-3/hour, 35 minutes, easy change at Slough; £10 one-way standard class, £10-14 same-day return, www.gwr.com). London’s Waterloo Station connects with Windsor & Eton Riverside (2/hour, no changes but slower--55 minutes; £10-13 one-way standard class, £12-20 same-day return, info tel. 0345-748-4950, www.nationalrail.co.uk).

Also note that tickets are not interchangeable between the two routes, ie on a return you can't say depart from Paddington and arrive back to Waterloo.

This company does Thames cruises in and around Windsor (not to/from London): https://www.frenchbrothers.co.uk/public-trips/