Faced this problem while i was installing drush with composer.
For anyone who have installed drush with composer, if this problem occures, You will have to set PATH variable as follows.
F:\Users\{YOUR-USERNAME}\AppData\Roaming\Composer\vendor\drush\drush\;
I did this and now my path variables are below.
F:\ProgramData\Oracle\Java\javapath;%SystemRoot%\system32;%SystemRoot%;%SystemRoot%\System32\Wbem;%SYSTEMROOT%\System32\WindowsPowerShell\v1.0\;F:\Program Files\TortoiseSVN\bin;F:\Program Files\Git\cmd;F:\wamp\bin\php\php5.5.12;F:\ProgramData\ComposerSetup\bin;F:\Users\user\AppData\Roaming\Composer\vendor\drush\drush\;
Be careful, Just don't copy paste above variables, you have to append drush path to existing variables.