You can find our manual at :
http://www.open-e.com/service-and-su...loads/manuals/

And you can see that Open-E system is an Operating System, not just a service or a tool.

So to install Windows 2003/2008 within it, that means installing operating system within operating system, and the only way to do that is by using a virtual solution such as VMware.
Then you can install all the kind of operating systems that you want within the VMware that is running on a Volume created by Open-E system.

You may now ask, so how to deal with VMware and Open-E, the answer will be found in one of the PDFs or videos within the following links:
http://www.open-e.com/library/webcasts-and-videos/
http://www.open-e.com/library/how-to-resources/