Normally I do install Laravel 5.1 by following this command regarding to documentation:
composer create-project laravel/laravel --prefer-dist
In addition to @WouterJ answer.
Worst case if the steps provided by @WouterJ did not work, you could manage to uninstall and reinstall composer for windows.
When done, run composer global update
to be sure to get latest updates, if there was.
Then run composer global require "laravel/installer=~1.1"
and it should works.
Remember to update your windows environment path C:\Users\UserName\AppData\Roaming\Composer\vendor\bin