Try this:
Make an archival backup copy of FS9.EXE
`1.) Open My Computer (aka: "Windows Explorer" ...not to be confused with Internet Explorer)
`2.) On the menu bar of that Windows Explorer session started via My Computer, click "View" > "Details" to enable a 2-pane view
`3.) Navigate/browse to the [FS2004 install path] folder (default is C:\Program Files\Microsoft Games\Flight Simulator 9)
`4.) In the right window pane, locate FS9.EXE, and "select" it with a single left mouse click (filename turns blue if selected)
`5.) Right-click the selected FS9.EXE filename and choose "Copy"
`6.) In the left window pane, "select" the above [FS2004 install path] folder name (folder name turns blue if selected)
`7.) Right-click the selected [FS2004 install path] folder name and choose "Paste"; press {F5} to refresh Explorer's view
`8.) In the right window pane, locate "Copy of FS9.EXE", and "select" it with a single left mouse click
`9.) Right-click the selected "Copy of FS9.EXE" filename and choose "Rename" (name is now "selected' inside a text frame)
10.) Type over the existing file name and use instead a name such as "FS9_EXE_Before_3GB.bak"
11.) Click in any white space outside the text frame to verify the rename was successful
12.) Close the My Computer > Windows Explorer session and return to the Windows Desktop.
Download "Explorer Suite" NTCore Editing Utility
`1.) Download "Explorer Suite"
http://www.ntcore.com/exsuite.php home page)
`2.) Run the downloaded "ExplorerSuite.exe" to install it, accepting all defaults
`3.) Click Programs > Explorer Suite > "CFF Explorer" to run that utility program ("CFF Explorer VII" window loads)
`4.) In CFF Explorer, click File > Open and browse to the [FS2004 install path] folder
NOTE: Default [FS2004 install path] is C:\Program Files\Microsoft Games\Flight Simulator 9
`4.) In the right window pane, locate FS9.EXE, and "select" it with a single left mouse click (file name turns blue if selected)
`5.) At the bottom of the CFF Explorer File > Open > "Select File" browse dialog box, click "Open"
`6.) Now back in CFF Explorer, one sees a 2-pane view with a virtual folder tree on the left, a table on the right
`7.) In the left window pane, locate NT Header > File Header and click "File Header" (table on right changes)
`8.) In the right window pane, locate the bottom table row "Characteristics"
`9.) In the far right cell labeled "Click Here", single left click that cell ("Characteristics" checkbox list dialog pops up)
10.) Add a "Check mark" to the check box "App can handle > 2GB address space"; leave all other check boxes unchanged
11.) Click OK, then Exit CFF Explorer.
