I'm going to Windsor next week but want to spend one day in London. I won't have a car. What's the best way to get there. Please be somewhat specific.

Train - couldn't be easier.

there are two train stations in Windsor. Trains from one go to Waterloo, and from the other, trains go into Paddington.

It depends on where you will be in Windsor which station is most convenient. Just ask anyone to direct you to the nearest station - very simple.

It also depends on where in London you want to wind up. Paddington is sort of northwestish; Waterloo is central on the south side of the river across from Whitehall.

"...across Westminster Bridge from Whitehall."

So if I didn't have a lot of time and wanted to hit more of the sites, the Waterloo destination would be best?

Yes - Waterloo would be more central. But if you want to see a LOT, the hop-on-hop-off open-air double decker tour buses have a stop right outside of Paddington Station.

So it all depends on what you are doing. Paddington is on 4 different tube lines, and Waterloo is on 3 lines so you can get to just about anywhere from either. The trains into Paddington take less time than the trains to waterloo.

There are two stations in Windsor. Windsor and Eton Riverside to Waterloo is direct and will take 45 minutes, Windsor and Eton Central to Paddington involves a change of trains in Slough, but will take 35 minutes.