I googled it, and seemed to find the answer, strange...

Anyways, it seems as if the Windowsbase.dll forms part of the .NET Framework 3.0 - well, it gets installed with it.
Also, if you install the Visual Studio 2005 extensions for .NET Framework 3.0 (Windows Workflow Foundation) you'd be able to utilise it.

This dll is mainly used to host the System.IO.Packaging Api

I hope my info was useful.