Your CPU supports NX but Windows 10 cannot detect it.
For example, you run the "Get Windows 10" app and it says this:
The CPU isn't supported.
...or you run the Windows 10 installer and it says this:
You can't install Windows 10 because your processor doesn't support NX.
This happened to me on one of my Core 2 Duo systems. I was absolutely sure that it supported NX.
The solution? I had to DISABLE NX / Data Execution Prevention in BIOS!
Once it was disabled the Windows 10 installation worked just fine.
After Windows 10 is installed you should probably go into BIOS and re-enable NX.