When you said that you tried to install FSX on Windows 7, did the install go through and you were able to launch FSX? Or did you have problems here? I'm assuming that you either had a dual boot situation on your computer, or you used another computer with Win7.
For your Windows 10 installation, you could try the general fix for Activation problems, but it could be that FSX is still recognizing your activation in Windows 7 as well. But you could try this on your Win10 machine anyway. Follow these details I've given, but be careful when amending System Registry values as putting a wrong entry or using the wrong key could create more problems for you. Back up the Registry first, if you're unsure. See this link for backing it up:
https://support.microsoft.com/en-us/help/322756/how-to-back-up-and-restore-the-registry-in-windows
After that, you will need to go back to your System Registry to try and fix the Activation problem, by following these directions:
- Close FSX and run Registry Editor (press Windows key + R and type regedit and Ok)
- Expand the following registry key:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Class{4d36e967-e325-11ce-bfc1-08002be10318} The letters in this key may be capital letters (e.g. 4D36E967.....) .
- After you find the folder, double-click Upperfilters on the right side.
- Delete any value there and type in PartMgr (note, this is case sensitive; type it as shown here) and click OK.
5. Close Registry Editor and reboot Computer.
- Start FSX and check whether the Activation box comes up.
Hope you have success with this. Let us know.
Chris