Stonehenge, Windsor Tour

We did an Even Evans tour on a bus and the guide had us laughing all the way. You can also search Viator.com. I don't have any info about private tours but I bet they are pricey.
Some will say to forget Stonehenge but I loved it! Been there twice.

See if International Friends have anything that might suit - small tours, often recommended. https://www.internationalfriends.co.uk/stonehenge-tours.html

I used Viator for a day trip out of London and was generally satisfied. I found them to be very reasonable as to cost. It's a nice tour bus and the guides are very good. My only complaint is that they try to pack in a little too much in one day. (Two events is good - three events is a little rushed, imo.) My memory is that they have a Stonehenge/Windsor Castle tour.

You could use one of the many excellent London companies doing day trips.
This company has several good day trips. Premium Tours:
https://www.premiumtours.co.uk/category/out-of-london-tours-uk-britain-england

Another very good company is International Friends. Here are some of their day trips out of London:
https://www.internationalfriends.co.uk/small-group-day-trips-tours.html

Two more companies doing day trips out of London:
www.EvanEvansTours.com
www.goldentours.com (Gray Line)

The least expensive company doing day trips out of London is the excellent London Walks Company.
www.walks.com
Take a look at their day trips with a guide.
You say you don't want to be on a big bus. London Walks Company will direct you to meet them at the train station. The guide will be there to greet you. All in the group will travel by train to your destination. Then the guide will give you a tour of what you came to see and then return to London with you.
On this page, click on the red bar that says "The Day Trips From London". Your Stonehenge trip is covered there:
http://www.walks.com/other-tours/day-trips-from-london/

Viatour, as mentioned by another poster, is a "middle-man". They do not actually operate the tours. They book you with a tour company, and add their fee for doing so. You want to deal directly with one of the other tour companies.

Windsor. The trip out by train from central London is very simple. If you want to do this, about two dozen of us here on this forum can tell you how. Windsor is just on the outskirts of London, near Heathrow Airport. Easily done by you and your travel companions.

However, if you want a guide, look at this page:
http://www.walks.com/other-tours/private-walks/

You could hire a guide from London Walks to do a private tour.

Email them: [email protected] and tell them what you are wanting to see with a private guide.
This may not be any more expensive than doing one of the big bus tours.

You could even hire the private guide to go with you to Windsor then out to Stonehenge.
This would all be done by train, except when you get to Salisbury, a bus takes you out to Stonehenge.
This you could also do on your own.

Thanks to all for the suggestions. I'll look into all of them. It sounds like it should be no problem seeing what we want. One company that wasn't mentioned by anyone is The English Bus. Does anyone have any experience with them? Also does anyone have any suggestions for a hotel close to Buckingham Palace? We are willing to pay more to be able to walk to some things but would like to keep it under $250/room and of course less if possible.