There are three versions available: previous, edge, and stable.
previous will use the previously released image, edge uses an image that is in preview, and stable uses the latest image. An image will be tagged as edge before it is moved to stable.
Monthly updates regarding the images are posted in this forum. Here is the post for January 2022:
[January 2022] Windows image updates available for preview on edge tags
Here are some additional links as well:
https://circleci.com/docs/2.0/hello-world-windows/#software-pre-installed-in-the-windows-image
https://circleci.com/developer/machine/image/windows-server#how-this-image-works
Please let me know if you would like further clarification on anything.
Best Regards