WinPrint is a simple command-line utility that takes two arguments:
the numerical handle of a window, and a filename to write to.

It will locate the window that you have specified, and then capture
the visible contents of the window and write it to a file as a bitmap
format (BMP) image.

This was compiled using Visual C++ from Visual Studio .NET 2003.

It requires Windows XP or higher to run, due to the use of the
PrintWindow() function, however it should be possible to rewrite it to
fallback to the use of the WM_PRINT command on systems without that.