>>I don't see it in this list (is it in System.Security ?), but I'm running it right now, so I would be able to see, if this is installed. Found unrelated problem in the meantime, though.
>
>No, it's in System.Web.

It is System.Web.Security.Membership.

When I try to go into the code I'm getting a JavaScript error from

// Name:        MicrosoftAjax.debug.js
// Assembly:    System.Web.Extensions
// Version:     3.5.0.0
// FileVersion: 3.5.30729.1
//-----------------------------------------------------------------------
// Copyright (C) Microsoft Corporation. All rights reserved.
//-----------------------------------------------------------------------
// MicrosoftAjax.js
// Microsoft AJAX Framework.