You need to append maven bin directory path to Windows %PATH% variable.
Control Panel -> System -> Advanced system settings -> Advanced Tab -> Environment Variables
-> The path variable is in the second section the *System variables*
.
Ensure you have the value ;%M2_HOME%\bin;
(note the semi-colon, different paths should be separated by them) in order for Windows to be able to find the Maven command definitions.
Ensure you restart your command prompt. Environment variables need to be set before you open your command prompt.