How to get from South Kensington to Windsor

Me personally, I would take the circle line 5 stops to Paddington, then the Elizabeth Line (a Maidenhead or Reading bound train) to Slough, change at Slough into the branch shuttle to Windsor and Eton Central.
Some people may validly say take a district line train 12 stops west to Ealing Broadway, change into the Elizabeth Line there. It'll be a bit slower doing that.
The Windsor shuttle train is run by Great Western, the vast majority of trains to Slough by Elizabeth Line. It doesn't matter- take the first train to Slough, same fare.
Elizabeth Line is part of TfL rail.
This is beyond Oystercard land, but the cheapest way to make the journey is by contactless.

The Castle doesn't open until 10am, and is not open at all on Tuesday and Wednesday. In any event it is much cheaper to start your journey after 9.30am, making 11.00 or 11.30 a good booking time.

Groupsave tickets for your party of 4 would be £33.65 return from Paddington. You can use these after 9:30 and return anytime later on which will beat contactless all the way for cost even taking into account the tube fare (especially if returning between 1600 & 1900).

The GWR service from the main line station runs non stop to Slough twice an hour as an alternative to the Elizabeth line - the final destination to look for on the boards will be Didcot Parkeay. The fares are the same.

Elizabeth Line changes everything! I haven't been there since it opened. In the old days, we would have said Tube to Waterloo, train to Windsor & Eton Riverside.

I would jump on a bus to Putney then take the train from there. It's barely longer than other options and is just one very easy change

Apparently the Elizabeth Line now accounts for 1 in 6 of all passenger rail journeys in the UK- currently running at an annualised figure of 240 million, which is 20% above best predictions one year after opening.
The route to W and E Riverside is still there. I would even argue a case for taking a District line train to Richmond and joining the Riverside train there. It's the route I always used to take to Windsor. And I still think it's better for having no change and more capacity on the trains, and in my view, more scenic.
Unfortunately I have learnt that on this forum Central is regarded as the only valid route (as it's faster), and that Riverside just gets shouted down every time you dare to propose it.

No one yet mentioned the direct 702 bus from Kensington High Street via Hammersmith? Oh I just did. There are many ways ...

Central means having to go through Slough. And if you've ever been through Slough, you know why you want to avoid going through Slough.

I just went on to the GWR website for tickets for tickets from Paddington to Windsor on June 2 -- this is the message I'm seeing:

"Industrial action to affect services on 2 June. This service may not run. Please check your journey closer to the date of travel".

On another part of the GWR website, it says "The Aslef union has announced a strike that will affect rail services May 31 and June 3 (not my travel dates), and the RMT has announced a strike on June 2 (this IS my travel day).  So I'm unclear if this means there will be no trains on May 31, June 2 and June 3??

I know there have been a lot of recent rail strikes. What should be my "Plan B" to get to Windsor that day?

Thank you all for your help!

June 2 is an RMT strike day. GWR and the Elizabeth Line will certainly be running a main line service that day as far as Slough. On the last RMT strike day the Slough to Windsor Shuttle ran as timetabled, every 20 minutes- the normal 2 car train.
Last train back was at 1905.

On the last RMT strike day South Western Trains (who run the service to Riverside Station) ran their normal 10 car train from Waterloo to Riverside every 15 minutes (I've just gone backwards and checked what actually happened that day, as opposed to what they said would happen, in case something went wrong that day, but it didn't).
Last train back was at 1805.

So you should be OK.

As already stated there is the Plan C Greenline Coach Service 702-

You will see that this stops at Slough Wellington Street (which is straight outside Slough Railway Station)- together with local service 2 and Greenline 703 they provide three buses an hour to Windsor.

As a Plan D there is also a Greenline Coach Service 703 which runs from Heathrow Terminal 5 Bus Station to Windsor

Thanks isn31c!

Also, should I buy train tickets on GWR in advance (like now!) given the strike, or do people typically buy GWR tickets from Paddington to Windsor at Paddington? I also wasn't sure if the strike would change the answer to this question.

Thanks again!

since the Paddington - Slough - Windsor & Eton Central journey can be done with contactless, many people (locals anyway) wouldn't buy tickets for that journey ahead or at all - just tap in and tap out. If you want to get the ticket ahead and risk the strike feel free, or you can get an actual or virtual ticket on the day, but simplest is just tap in and tap out...

The price will not change in advance or on the day.

As we've established above a 'groupsave' ticket is cheaper than contactless.

That can easily be purchased on the day from the machines or manned ticket windows at Paddington. When you put 4 people into the machine it should automatically offer you a groupsave fare. Use a manned window if the machines are awkward.

Or even buy the groupsave on your phone as a virtual ticket (or a collect from machine physical ticket) as you eat breakfast at your hotel that morning.

Any ticket purchased on line can be collected from a machine 15 minutes after purchase.

Note that the Greenline routes 702 and 703 which I mention above still run, and to the same timetables.

However they are no longer part of the Greenline network. They were and are run by Reading Buses who have just purchased brand new luxury double decker buses for the routes.

The London route has now been renamed as The London Line 702 and the Windsor route is about to be renamed as Flightline 703- the vehicles on the 703 route will include large on board stowage areas, whereas the 702 is mainly a commuter route.

I have no skin in the discussion about whether to take the train from Paddington or Waterloo to Windsor. But please note that due to engineering works there are no through trains from Waterloo to Windsor from 19th August to 1st September.