Like Joseph Ottinger said, you have to put Maven installation directory to your PATH environment variables or User variables. You can edit user variables by running command "rundll32 sysdm.cpl,EditEnvironmentVariables" without quotes.
First add new variable called MAVEN_HOME and for it's value Mavens installation directory ie. c:\tools\maven-3.0.3\ then edit PATH variable and add end of the value ;%MAVEN_HOME%\bin (notice semicolon)
After re-login or opening new command prompt mvn command should work perfectly.
