Mach4 is almost Windows 10 compatible... at least for me. It runs great the first time after installation, but won't restart after once exiting it. Windows says the task is running... but Mach is not visible on the screen... I just see the desktop as it was before I started Mach. If you try to start it again, you get the message that Mach is already running.
Solution: Before you start Mach the 1st time (or just restart Windows) right click on the Mach4Mill (or whatever flavor you're using) icon on your desktop, and select "Troubleshoot compatibility". Windows will analyze the Mach4 program. Then select "Try recommended settings". It will recommend Windows compatibility mode: Windows 8. Select "Test the Program". Mach4 comes up just fine (a couple loading warning messages may be seen... but they say the operation completed OK). Now exit Mach4, and you are back on the troubleshooting window. Click on "next", and select "Save these settings".
Now just click on Mach4Mill, and it starts up normally every time. Hope this helps somebody else!