Please see the most recent updates in the "Where did the .com name go?" thread. Posts number 16 and 17.
It is very difficult to completely remove FSX from the registry and if you don't,
FSX Steam will use external folders named FSX_SE instead of FSX.
There is a registry fix for this however.
The recognised fix for g3d.dll errors is to install FSUIPC, whether registered or not
it removes them for most people.
The link's down at the moment.
This would only apply if FSX has not been completely removed.
Changing the registry entry works the next time that FSX Steam is run and
is entirely reversible.
The way to do it is to navigate to
HKEY_CURRENT_USER\Software\Microsoft\Microsoft Games\Flight Simulator - Steam Edition
where you will find a key named Co-Existence.
Its value by default is 1, which makes FSX Steam create the external FSX-SE folders.
If you change the value to 0, FSX Steam will create external folders named FSX.
In this way, installers built for FSX will find the files in exactly the right places and
work properly.
The few installers that detect the FSX root folder using the registry and do not offer the
choice to navigate elsewhere will require the existing (64 bit OS) FSX location at
HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\microsoft games\Flight Simulator\10.0\SetupPath
to be amended to match the new FSX Steam location.
All of my addons (I have allot of them) work great. So do not let the stand in your way.