1

The Heathrow express is the best bet, It takes 15mins from Heathrow to central London, To paddington station
https://www.heathrowexpress.com/

It does cost around £35 return but it is by far the quickest and most efficient and reliable service.

As for windsor it is a direct train from paddington but takes 40mins to get there, so it would be a no go for your 5hr layover but very probable for your 12hr one, aslong as it is during the day. Windsor is not on the tube line but is connectable by an overground normal train

Hope this helps

2

Refer to http://www.tfl.gov.uk - it has an excellent search engine. The exact advice depends on which terminals you're arriving into and departing from. Set your search for the specific terminal, then search for windsor castle as a place of interest.

You should really be back at Heathrow no later than 1.5 hrs before your next flight takes off, just in case security has gone bananas again - I've queued for 45 minutes before.

Alternatively, if London grabs your fancy, take the HEX as #1 mentioned, or if on a budget take the tube. If you're looking for London in general, consider taking the picadilly line all the way to Covent Garden and figuring it out from there. (basically head down the hill through the market area to the Strand (theatres), turn right for Trafalgar Sq, then either continue walking or hop on a hop on/off bus to get a mega tour in a hurry around the capital. Links to reputable tour bus operators are in the FAQ or on http://www.visitlondon.com.

Taking the 1.5 hr arrival before your flight advice again, you wanna be departing from central London about 2:40 before your scheduled departure from Heathrow, unless taking the HEX, in which case you wanna be navigating from central London to Paddington about 2:15 before your scheduled departure from Heathrow

3

Windsor is very close to Heathrow: no train or tube, but a short bus ride. You could probably visit during your six-hour visit.

4

Voyager - it's actually an hour's bus ride. And there are about four buses per hour from Terminal 5. However, it's better than sitting round in a terminal for six hours, even if you don't get much time there.
Go for it. (The slightly longer journey is the most scenic.)

5

Thanks all! And thanks for the links yorkshire and fwoggie. Things are coming together.

6

Try this website for information about buses to Windsor. If you haven't been there before, I would definitely recommend it. Even if you don't have a chance to see the castle itself, the town is very nice to wander around. And the long walk leading up from Windsor Great Park does give some very nice views of the castle as well.

http://www.londontoolkit.com/whattodo/windsor_heathrow_bus.htm