We running Windows 7 on all of our production machines for many years and never had issues.
All updates are disabled, so no surprises happen.
XP is also a good choice though.
And Win 10. will also work, but I personally don't like that you cannot disable updates. I mean you can on the non Home versions, but after some time Windows will switch it back on automatically.
I also don't like that updates are coming very frequenty and they often contain serious bugs which are then not corrected for months or for years.
A good example is the OpenGL bug which we had to find a workaround for which made some graphics cards not to run some OpenGL functions and hardware graphics acceleration to work properly.
This bug as far as I know is still not corrected and it had been released through the Windows updates a bit over one year ago.
So my personal opinion is that XP and Win 7 are currently better choices for controlling a CNC than Win 10.