Just like Fmarano said, it could be the ABS sensor. The later model Windstars have also been notorious for problems with the ABS modules. And trust me, I work in Ford parts and that is a very expensive fix if it is indeed the module itself.
We used to own a Windstar, and the ABS light went off and on all the time. We drove it for over a year like that without any problems, until the engine blew a rod. :wacko: