Like others I've been hoping that either the turbotax experts would tell us HOW to make Win7 work not just that it will work, or that community experts would find a straightforward procedure and let us know. As it happens I found a trivial solution that works on my computer, and hope it works for others.
Go to the downloaded turbotax exe file, right-click, and choose Properties.
At the bottom of the properties box there's a statement that this file came from another computer and might be blocked to protect your computer. Beside it there's an Unblock button. Click it and everything works!