error while installing composer

i am trying to use symfony2
i downloaded symfony package,then composer for windows7 , i also downloaded php 5.3.2 to complete installation of composer after finishing i used command prompt to run composer
i changed directory to composer directory and wrote this command

composer install 

i found this error

run time exception
failed to clone http://github.com/kriswallsmith/assetic.git,git was not found ,check that it is installed