Occasionally you may get an error that a problem on Windows is "not a valid Win32 application." This message may seem confusing, but it simply means that your version of Windows isn't able to run the app since it can't understand what's in the file. To fix it, reacquire the file from the place you got it. If that doesn't work, you may need to use another Windows or even MS-DOS version to run the file.
What's a Win32 Application?
Win32 is Microsoft's name for the programming interface for the 32-bit versions of Windows, meaning programs access data in blocks of 32 binary digits and can use the same amount of space to refer to sections of memory.
Earlier versions of Windows and the preceding operating system, called MS-DOS, were 16-bit operating systems, and 64-bit versions of Windows are now available. Generally, 32-bit versions of Windows can run 16-bit programs as well as 32-bit programs, and the 64-bit versions can run 32-bit programs and 64-bit programs.
Trying to run a non-Win32 application on a 32-bit version of Windows or even a 64-bit version of Windows can trigger the Win32 error.
Not a Windows 32 Application
If you receive a message that a program is not a Win32 application or something similar, there are a number of possible causes.
One is that the program is for a newer or older operating system than you're running. Another is that the file itself is somehow corrupt or just incompatible with Windows.
If you try to run a macOS or Linux application on Windows, you might see that error. You might also see it if you try to run another type of file, such as a Microsoft Word document or an image, as if it were an application, which can happen if the file extension is incorrectly set to ".exe."
You may also see the error if a program was improperly installed, leading to data corruption. If you downloaded a program or installed it from a disk, try reinstalling it and see if the error goes away. Contact the program maker for help if you need to do so.
If you compiled the program yourself from source code, whether it's an open source tool or one you built yourself, check your compiler settings to make sure you're building for the right Windows version.
Emulation and Old Windows
If an older program won't work on your version of Windows, one option is to run it in an emulator or virtual machine.
You can use a tool like VMWare or VirtualBox (See Resources) to install another, older operating system that might be more compatible with your program. It will run in a window on your computer so you can effectively run it on top of your version of Windows. You will usually need an installer for the operating system.
You can also use a Windows or DOS emulator to run older programs. DOSBox can be used to run DOS programs in an emulator, and FreeDOS is a free compatible alternative to MS-DOS. You can run Windows programs on Mac or Linux machines using a tool called Wine (see Resources).
You can also run another version of Windows or DOS on another computer if you have one.