'windiff' is not recognized as an internal or external command, operable program or batch file

WinDiff is not a command-line tool, it is a windowed program (though it does take command-line arguments).

Furthermore, it is part of Visual Studio, not Windows. However, you can also get it in the Windows XP Support Tools.

It’s pretty easy to use, but you can get more out of it by setting the options correctly (eg: uncheck the Ignore Blanks option and set the editor, preferably one with a jump-to-line command-line argument).

In addition, there is a good front-end for it as well as a shell-extension.

Windiff is a not a part of a standard Windows installation. The MS Version used to come with Visual Studio. I dont know if it still does. However, you can always download Visual Studio Exrpess to see if its there. Or there are tons of free diff programs on the internet.