Reformatting the drive where FSX was installed won't change anything in your registry keys. You should have UNINSTALL FSX first, in order to clear all your old registry keys, which are now probably messed up.
I suggest to do the following:
- Uninstall FSX Steam
- Remove the following folders:
%APPDATA%\Microsoft\FSX
%APPDATA%\Microsoft\FSX-SE
- Open the REGEDIT utility and search your registry for all the FSX registry keys. Remove them ALL, both the FSX and the FSX SE keys.
- Reinstall FSX Steam