I had thought MAW was a difficult installation through v1.31. The "new" ETO surpassed its adventures in computer patience. I had ETO from last November as encouraged by MAW members . No problems at that time. Yet as my OS died, was restored and migrated from SP2 to SP3, things became more complicated .
I had early problems with ETO 1.0 due to a cockpit error where I failed to note the option selection on the installer. Without checking the continue message, the installation ended abruptly without an installation. My third attempt discovered this "oversight"and success was achieved. But...
I installed v1.20 without hitch, loaded and launched the initialization run at Level 1 (1936), flew a plane to secure the initialization setup and then moved to step 3 or Firepower.
While I already have v1.08 of Firepower in CFS3 and the ETO v1.0 setup did copy those files, I proceeded with the Firepower ETO patch. After a system reboot, a selection of Level 1, the program went immediately on two attempts to a CTD following the introductory CFS3 video.
Recopying, no clean "do over", v1.0 and v1.2, the program now launches and checks out to Level 3 (1943)
In the end I still have reports on launch of SDK violations due to a marginally constructed plane imported into CFS3 at some time, a wandering DPC Korea F51, "gun" and "searchlight" files already in use. These I can bypass. I did have to eliminate the Tempest MkV as it caused a freeze following the CFS3 video. Moving this plane folder from Aircraft eliminated this problem. Annoyances aside, the program now functions, without the Firepower patch. Now comes the careful addition of planes, Spawns, Missions that will not lead to another file conflict or program breakdown in a fragile program setup. At least I now know that tenacity, sometime brute force recopy in lieu of "clean copy" can eventually restore functionality.
Over

Phil,
ETO was never test-installed over FirePower. A lot of the specifically-modded ETO files will destroy some FP files.
FirePower should not be installed over ETO for the same reason.
Here's your safest and quickest bet:
1. Delete everything: both CFS3 (Including FP) and ETO installs.
2. Reinstall CFS3, update fully (3.1 and 3.1a), then start it up and set everything the way you choose. Thenm shut the game down.
3. Install CFS3 ETO Expansion 1.0 per installation instructions.
Note: Be sure to not lose all five icons that will be set onto your desktop somehow. Personally, I make a folder on the desktop named CFS3 ETO Expansion. Then put those five icons into it.
4. Start CFS3 ETO Expansion 1.0 and set everything the way you want it.
5. Now make sure that ETO is started in Era 1 and exited. Then set the Spawn Selector to 1, Stock Spawn Mode.
6. Install CFS3 ETO Expansion Cumulative Update Rev 1.20. Start it up and make sure everything is set properly.
7. Now install you FirePower into your stock CFS3.
8. After that, go into Program Files/Microsoft Games, and look for your Combat Flight Simulator main game folder and simply re-name it to reflect that it is now FirePower.
9. Now go into your CFS3 ETO Expansion main game folder and look for a file named "MultiCFS3.exe". Then copy it (do not cut and paste, but copy and paste only) into your new FirePower install and run it. This will enable you to create two new folders in your Application Data/Microsoft and My Documents folders. When you run the FP install next, those two new folders will be created in the Application Data/Microsoft and My Documents folders.
10.. Now it is actually safe to install stock CFS3 again, and update it, of course. Now you have a comp[lete FirePower install, an ETO full 1.20 install, and a new vanill CFGS3 install.
11. Now go into your FirePower install anda copy these four folders into your CFS3 ETO Expansion main game folder's aircraft folder: B_17_share, b_17f, B_17G, and B_24J.
12. Make sure ETO is set to Era 1 and exited, and the Spawn Selector is set to 1, Stock Spawn Mode. Now install FirePower_USAAF_Heavy_Bombers_2eto.exe.
You should be good to go now. Good luck and have fun.:d