For a plugin for Sublime Text I was required to install a composer package globally.
In order to do this I had to run the following command:
composer glo
You can query Composer to find where it has set the user $COMPOSER_HOME
directory.
composer config --list --global
The '[home]' line refers to the default value of $COMPOSER_HOME
.
Just as a final confirmation - the 'yourname' part in the configuration, would very likely need to be updated with your own Windows username - composer config
will say where it put the files.