<p>I am very interested in learning a Ruby on Rails / Django type web technology but am currently using Windows Server 2008 (for .Net web applications.) I have read that RoR is not very performant on Windows.</p>

<ol>
<li><p>Would it be unadvisable to create run a beta web application in RoR on a Windows Web server as far as performance is concerned?</p></li>
<li><p>Would Django or another similar technology be a better choice? Can you be a "happy, productive programmer" and still use a Windows Server?</p></li>
</ol>

<p>(Note: I actually do enjoy heavily modified Asp.Net / C#. I'm looking to try some different stacks that might be fun and productive from the start.)</p>

You can still learn a technology on Windows even if it isn't good at high performance on that platform yet.
I strongly recommend
against
trying to learn new technology at the same time as implementing an actual project you care about.  It's better to write a pretend project while learning new tools or languages.
That way you can try out features of that technology that aren't relevant to your desired project, and you aren't distracted by focus on your own project's requirements while you're trying to learn.
So go ahead and learn both, experiment with them on Windows.  There are good step-by-step tutorials available and ready-to-install bundles for both
Rails
and
Django
on Windows.