Copied from Phenozo's sticky over at SOH.com:


ETO for Win7 with WinXP Host

CFS3 ETO Multiplayer Fix for Windows 7 64-Bit joining to a Windows XP Host

There was a solution found for Windows 7 users of CFS3 to connect online without having "aircraft mismatched errors" in stock (3.1a updated) versions of the sim which I then tried to apply to the trouble I was having in connecting to "ETO Expansion v1.20 add-on in use" multiplayer games.

After troubleshooting it was discovered that there indeed was a compatibility issue reported between Windows XP and Windows 7 users not being able to connect when using the CFS3 ETO Expansion online.

This can be overcome in a similar way as the Windows 7 stock CFS3 Multiplayer Fix is done, but with additional files which need to be edited.
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Step One :
Go into the Main Folder of the CFS3 ETO Expansion and locate the cfs3.exe

C:/Program Files (x86)/Microsoft Games/CFS3 ETO Expansion

Right-Click Copy the cfs3.exe, then Right-Click Paste it onto your desktop
Right-Click Rename it, CFS3etoWin7.exe

Next Right-Click Cut that new .exe from your desktop and Right-Click Paste it back into the CFS3 ETO Expansion's Main folder located as stated above.

Next Right-Click that renamed .exe and select Properties :

Select the Compatibility tab

Compatibility Mode :
Check the Box for Run this program in compatibility mode for :
In the drop down menu select Windows XP (Service Pack 2)
Priviledge Level :
If you can, tick the box for Run this program as an administrator also.
Then click the Apply Button and then the OK Button

Step Two :
Go into the Main Folder of the CFS3 ETO Expansion and locate this file
MASTERETOSTARTUP.bat
Right-Click Copy the file, then Right-Click Paste it onto your desktop
Right-Click Rename it, MASTERETOSTARTUPwin7.bat

Next Right-Click Cut that new file from your desktop and Right-Click Paste it back into the CFS3 ETO Expansion's Main folder located as stated above.

Next Right-Click that renamed .bat you just pasted and select Properties :
Select the Compatibility tab
Compatibility Mode :
Check the Box for Run this program in compatibility mode for :
In the drop down menu select Windows XP (Service Pack 2)
Priviledge Level :
If you can, tick the box for Run this program as an administrator also.
Then click the Apply Button and then the OK Button

Step Three :
Right-Click that new MASTERETOSTARTUPwin7.bat and select Edit
You will see these lines :

@ECHO OFF
CLS
CALL ETOEraSwapper.bat
CD "C:\Program Files\Microsoft Games\CFS3 ETO Expansion\global_layer"
CALL ETOGLSwapper.bat
START vistacfs3.exe

You will need to correct two of them :

@ECHO OFF
CLS
CALL ETOEraSwapper.bat
CD "C:\Program Files (x86)\Microsoft Games\CFS3 ETO Expansion\global_layer"
CALL ETOGLSwapper.bat
START CFS3etoWin7.exe

The first edit corrects the pathing and the second redirects to the new .exe

Once you have made these edits, save the changes when you close the file.

Step Four :
Right-Click the ETO Start Icon on your desktop and then select Properties :

Then select the Shortcut Tab :

And in the Target field :

"C:\Program Files (x86)\Microsoft Games\CFS3 ETO Expansion\MASTERETOSTARTUPwin7.bat"

Change it to match your new MASTERETOSTARTUPwin7.bat

Then click the Apply Button and then the OK Button

Step Five :
Right-Click the ETO Start Icon again and select Properties :
Select the Compatibility tab
Compatibility Mode :
Check the Box for Run this program in compatibility mode for :
In the drop down menu select Windows XP (Service Pack 2)
Priviledge Level :
If you can, tick the box for Run this program as an administrator also.

Then click the Apply Button and then the OK Button

This will now start the MasterEtoStartUpWin7.bat from the ETO Start desktop icon, which will then call on the "global layer swapper", and finally, launch CombatFS3 ETO with the compatibility correctly set for Windows 7 to be hosted by a user of Windows XP.