64-bit Windows: Unsupported 16-Bit Application Fix

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While installing a line of business application for a client who was upgrading from Windows XP (x86) to Windows 7 (x64), I was presented this dreadful error message:
"The version of this file is not compatible with the version of Windows you're running. Check your computer's system information to see whether you need an x86 (32-bit) or x64 (64-bit) version of the program, and then contact the software publisher."
After doing some troubleshooting, and of course, some Google'ing, I came across many people with a similar error trying to do the same thing:
"Unsupported 16-Bit Application
The program or feature "\??\C:\Path\To\Program.exe" cannot start or run due to incompatibility with 64-bit versions of Windows. Please contact the software vendor to ask if a 64-bit Windows compatible version is available."
Knowing that the software vendor had been out of business for the past 5 or so years, it was probably not worth trying to track him down to ask if he had secretly made a 64-bit version that he didn't release.

Of course, there were the standard fixes to this problem, which were:
Neither of these options were viable. Installing the 32-bit operating system would mean undoing all of the work I had already done on this machine, and XP Mode (for reasons I won't go into) would not work.

I continued to look for a better solution and that's when I stumbled across a thread on the ReactOS forums which said that these programs were often 64-bit compatible, it was just the installer that was 16-bit, and thus incompatible with 64-bit Windows.

So, if the file _inst32i.ex_ is in the program installation folder, we're in luck as this is an InstallShield based installer that we can get to install using the following steps:
  1. Make sure all of the setup files are in a writeable folder (not on a CD, for example)
  2. Right click on setup.exe, select "Properties", under the "Details" tab check the "File Version"
  3. If the File Version is 3.x, download this file. If the File Version is 5.x, download this file
  4. Extract the setup file from the downloaded ZIP to your program setup directory, overwrite if necessary
  5. Execute the setup file we just extracted and it should launch the installer

That was all - the program installed without any problems and life was great. The thread post I linked contains alternative instructions for other types of installers which I don't believe are completely relevant to Windows x64, but if you're still having problems feel free to take a look and let me know how you made out.