After a couple of days in London, we plan to take the train to Windsor and hire a car there. After touring the castle, we plan to drive to Salisbury. Questions: is there a car rental near the train station and can you stow your luggage in the car before taking the castle tour?
The only rental agency in Windsor seems to be Practical Car Hire - at 103 Arthur Road. That is on the edge of town, maybe 10 to 15 minutes walk from the Central station, a bit further from Riverside station.
Other rental offices are in Slough (where you change trains coming from Paddington)- Hertz, Enterprise, Avis and Europcar- all will need a short "taxi" ride from Slough railway station.
Alternatively you could consider renting from Heathrow Airport.