Windsor is easily reached from London by train, with two terminals in the town. Windsor and Eton Central is served by South Western Railway trains from Slough — change at Slough from a Paddington train (about 30 minutes total). Windsor and Eton Riverside has direct services from Waterloo (about 55 minutes). Both stations are a few minutes' walk from the castle.
By road, Windsor sits just off the M4 motorway between London and Reading (junction 6). Driving from central London takes about 45 minutes outside rush hour. Parking inside the town is metered and expensive; the bigger car parks (Alexandra Gardens, Romney Lock) charge a flat day rate.
Heathrow Airport is just 10 kilometres east, reachable by bus or taxi in about 20 minutes.