Can I install FSX and FS2004 on the same PC?

Yes. Microsoft Flight Simulator X (FSX, including FSX: Steam Edition) and Microsoft Flight Simulator 2004: A Century of Flight (FS2004, also called FS9) can coexist on one Windows PC. They use separate executables, settings and scenery libraries. Install them in different folders, patch each edition correctly, and keep add-ons matched to their intended simulator.

How should I install FSX and FS2004 together?

Install order does not matter, but completing and testing one simulator before installing the other makes faults much easier to isolate. You do not need to remove a working FS2004 installation before adding FSX, or vice versa.

1. Create separate installation folders. Suitable locations for boxed editions include C:\FlightSims\FS2004 and C:\FlightSims\FSX. Installing outside Program Files avoids many permission problems with older add-on installers.
2. Let Steam manage FSX: Steam Edition. Select an appropriate Steam library during installation and use Steam's storage controls if the simulator must be moved. Do not drag its folder elsewhere in File Explorer. If you are unsure which directory is active, follow our guide to finding the correct boxed or Steam FSX folder.
3. Install and initialise FS2004. Launch it once, create or load a default flight, then close it normally. Apply the FS2004 9.1 update where appropriate and test the unmodified simulator again before adding aircraft or scenery.
4. Install the correct FSX edition and updates. Boxed Standard or Deluxe without Acceleration uses SP1 followed by SP2. Acceleration installations have their own update path, covered in our boxed FSX and Acceleration setup notes for modern Windows. FSX: Steam Edition already includes the relevant service-pack changes and Acceleration content, so do not apply boxed patches or the boxed Acceleration expansion to it.
5. Test both shortcuts. Label them clearly and confirm that one opens fs9.exe while the other opens fsx.exe. Complete any boxed FSX activation before installing add-ons.
6. Add content one simulator at a time. Read every installer's destination path before accepting it. A path ending at C:\FlightSims is not sufficient when both simulator directories are beneath it.

FSX vs FS2004: will their files or settings conflict?

FSX and FS2004 do not normally conflict because each simulator maintains its own executable, aircraft structure, user configuration and scenery database. Graphics settings, control assignments and scenery priority changed inside one simulator do not alter the other.

The usual overlap is an old third-party installer reading the wrong registry entry or defaulting to a previous path. Run both simulators once before installing add-ons, and manually select the folder containing the correct executable whenever the installer permits it.

If choosing between FSX vs FS2004 rather than keeping both, let add-on compatibility decide first. FS2004 remains useful for an established FS9 library; FSX is the better match for content built around SimObjects, FSX gauges, missions and FSX scenery features. Both are legacy 32-bit Windows simulators, so installing FSX does not upgrade or replace FS2004.

Can I use the same aircraft and scenery in both simulators?

No—treat every aircraft, gauge, module, scenery package and AI traffic file as version-specific unless its documentation explicitly supports both simulators. Sharing a PC does not make FS2004 add-ons compatible with FSX.

• Aircraft: FS2004 port-over models may load in FSX but still have missing gauges, broken animations, visual defects or incompatible sounds. Check the compatibility requirements for using FS2004 aircraft in FSX before copying any files. FSX-native aircraft generally cannot be used in FS2004.
• Gauges and modules: Never merge complete Gauges or Modules folders between the two installations. Incompatible DLL-based components can prevent the receiving simulator from starting.
• Scenery: Some simple FS2004 scenery may partially display in FSX, but airport, terrain, object and texture differences can produce missing buildings, incorrect elevations or duplicated features. Keep separate copies even when a package provides files for both versions.
• AI traffic: An FS2004-format traffic BGL installed in FSX can stop FSX-format AI traffic from appearing. This is easy to misdiagnose as an aircraft or traffic-percentage problem.

What problems occur after installing both simulators?

Most failures are caused by a wrong path, protected Windows folder, legacy disc check or incompatible add-on rather than a conflict between FSX and FS2004.

• The installer cannot find a simulator: launch that simulator once to initialise its user files, then point the installer to the directory containing fs9.exe or fsx.exe. Very old installers may not recognise FSX: Steam Edition automatically.
• An aircraft does not appear: confirm that it went into the correct simulator and is not nested one folder too deep. FS2004 aircraft belong under Aircraft; FSX aircraft normally belong under SimObjects\Airplanes.
• Settings will not save or an installer reports Access denied: this commonly results from a boxed simulator installed under Program Files. Reinstalling into a dedicated writable folder is safer than disabling User Account Control for the entire PC.
• The wrong simulator opens: recreate the shortcut directly from the intended executable and rename it clearly. Also check shortcuts installed by add-on utilities, which may still target an old location.
• An Insert Disc message appears: verify that the shortcut points to the edition you own and that the required boxed media is present. FS2004's legacy disc-check technology can also fail on newer Windows versions; our FSX and FS2004 disc-error checks separate that problem from path and Steam-edition mistakes.
• FSX or FS2004 stops launching after an add-on: remove the most recently added aircraft, gauge, module or scenery entry and test again. Do not copy the same suspect files into the other simulator.

Can FSX and FS2004 run at the same time?

Yes, but running both simultaneously is rarely useful and is less stable than running one at a time. They compete for processor time, memory, sound output and controller input; joystick assignments may also respond in both windows.

Close one simulator before launching the other when testing add-ons, changing controller profiles or troubleshooting crashes. Their separate installation folders allow coexistence, but they are not designed to share a running flight or synchronise settings.