ok I dont use the Webservice but from PS documentation
http://doc.prestashop.com/display/PS16/Web+service+tutorial
I think you did all of this but just checking
To test if you have properly configured your access to the web service, try to access your online shop with the following URL: http://mypasskey@example.com/api/, where mypasskey is replaced with the key you created, and example.com with your shop's URL. Mozilla Firefox is the preferred browser to test your access, but any browser able to display XML should do just fine.
Do you have php-curl installed
type at cmd prompt
php -v
and
php -m
is curl in this list?
since CWP complies php from source you might need to make sure it's in there.
Also it could just be a general comment but
I see this
http://doc.prestashop.com/display/PS16/Web+service+tutorial
Prerequisites
PrestaShop 1.6 installed on a server with mod_rewrite enabled (Apache only).