composer path used in windows environmental variables is:

C:\ProgramData\ComposerSetup\bin

This is default path of composer on windows machine when using windows installer.

Useful to set up PhpStorm command line tool support for composer, you need only paste this to configuration:

C:\ProgramData\ComposerSetup\bin\composer.phar

screenshot: http://screencast.com/t/43dYedLr6y1s