APEX

I want to create a Web Service in Apex that can create appointments in my exchange calendar. Exchange is offering web services for that.
When i try to create the web service reference in Apex i provide the URL and my Exchange login, but it is failing.
The exchange guys told me, that there is a ISA Server running where i need to authenticate through a form.

Does anyone know, how i can implement a webservice in Apex, when the access to the wsdl url needs login through a form ?

Or : what other types of authentication does Apex know ? What possibilities do i have ?

Thanks for answers.
Bernd