Generally speaking employees don't really per se use windows so much as click the browser icon and proceed to use employers web based tools.

There's a bunch of non-web proprietary software medical offices use to access patient files, result histories, prescription dispensation etc. At least here in Ontario my doctor uses an actual windows application to accomplish all that.

Then they use those apps. The point is that since they usage of the OS as such is so minimal as to be irrelevant as long as it has a launcher and an X in the top corner.

They could as well launch that app in OpenBSD.

Momentum as well. Many of these systems started in DOS. The DOS->Windows transition is pretty natural.