WeBWorK is a client/server application. The server typically runs some form of unix (see installation instructions at http://webwork.maa.org/wiki/Installation_Manual_for_2.4#Installation_instructions_for_specific_operating_systems
On the client side for students and instructors using WeBWorK nearly any computer platform or web browser will connect to the server. There are no restrictions. WeBWorK courses can be hosted on servers anywhere on the net -- they don't have to be hosted at the institution offering the course.
It is not impossible to set up a webwork server on a windows machine -- it requires that apache and perl and mysql be installed and the installation procedures for those applications may be different from unix machines. I think some experimentation would be needed. I am not aware of any one who uses a windows machine to serve webwork.