We conclude a RS tour in Bath in September. After a couple of additional days in Bath, we are traveling to Windsor for a few days before we fly home. What is the best way to get from Bath to Windsor? We are not opposed to using a car service.
Train. You can do it in about 2 hours - express (125mph) train to Reading where you change to another (stopping train) to Slough and then change again for the short journey down the branch line to Windsor & Eton Central.
It is cheaper if you go on the 10.13 or later from Bath. An Advance ticket today for today is £40.30 but if you pre book Advance (specific train) further ahead, you can get prices around £30. (Those prices are without a Railcard discount).
If you are not opposed to using a car service, then the car service will be the easiest. Door to door. No changes. No lugging bags.
Sorry, I can't personally recommend a car service as I have no experience using one between these two points.