Messbox

Messbox is a really tiny (3KB) Windows program for displaying graphical message boxes from the command line. It is especially useful in batch scripts.
No additional files are needed to run the executable, it is entirely self-contained.
Messbox should work on any Windows from Windows 2000 Professional up to the latest versions.

To display a message, run messbox "your message". Don't forget those quotes if the message has spaces in it.

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Sourcecode

To build from source, you need NASM and some kind of linker.
Assemble using NASM and then link with kernel32 and user32 as a Windows GUI program with entry point NoCRTMain.
Example with the MSVC linker:
nasm -fwin32 messbox.asm
link -nologo -subsystem:windows -nodefaultlib -entry:NoCRTMain messbox.obj kernel32.lib user32.lib

(Linking with a C standard library is not recommended, as it will increase the size of the executable. The -nodefaultlib option for the MSVC linker prevents this.)