Apache Maven install "'mvn' not recognized as an internal or external command" after setting OS environmental variables?

I've followed the official installation instructions here for Windows XP. But sometimes when I execute `mvn --version`, I receive the error message, 'mvn' not recognized as an internal or external command I've even rebooted my machine a couple times, but the OS does not always recognize the maven command. But when I execute either, `cd %M2_HOME%`, or `cd %M2%` it brings me to the installation directory. I've also made sure to add the user variable `M2` to the `Path` system variable. So, what gives?