What is the cheapest way to do Windsor Castle?

I went to Windsor last November. It is only 8 pounds per person round trip from Londons Paddington Rail station to Windsor and back (change trains in Slough, not difficult). It is maybe 25 minutes total each way. Eton Central station is a bit easier than Eton Riverside as Eton Central puts you right in the middle of the town. Just take a small map and youll find your way around, as it is a small village rather than a city. The castle cant be missed, along the high street and on a hill. A very nice and fun day out. A tour is not necessary. I printed a map of Windsor off Google.

It was a fun day trip, though I could have spent a couple days there because there are so many fun shops and old buildings to see. Be sure to visit the "crooked" house built in the 1500s!

As Tami said take the train from Paddington Station to Windsor. When you get to Windsor and exit the train turn to your left and walk up to High Street and the Castle is right there, it's a 3 minute walk.
While in Windsor you can walk to Eton (five minute walkd) and to Windsor Great Park (5 minute walk). It's a great place to visit...enjoy!.

The cheapest transport is the Green Line bus from London Victoria to Windsor

If you use the South West trains from London Waterloo to Windsor instaed of the train from Paddington the train company often has bundles which give a discount on Windsor Castle.

If you are visiting other Royal Palaces like Hampton Court, Tower Of London, Kensington Palace you can buy combo tickets that give a discount.

You want cheap? Do what I did back in college. I took the train and walked all around Eton and saw the Castle. I didn't pay for a thing, and I thoroughly enjoyed myself. You don't have to pay to see every inch of a place to get something out of it.

The directions Robin gave is from the Eton Central station. There are 2 stations, but the Riverside one is not as central.