I've read the global installation documentation for Composer, but it's for *nix systems only:
curl -s https://getcomposer.org/installer | php
sudo mv composer.phar /usr/local/bin/composer
Sure. Just put composer.phar somewhere like
C:\php\composer.phar, then make a batch file somewhere within the PATH called
composer.bat which does the following:
@ECHO OFF php "%~dp0composer.phar" %*
The "%*" repeats all of the arguments passed to the shell script.
Then you can run around doing
composer update all ya want!