I found this via Gerhard Helzel's site.
Classical Text Editor is apparently a well-established and long-maintained Windows program that recently added OpenType support, despite the fact that it's written in... Delphi?!?!! I'm impressed!
I am trying it now and having trouble getting the OpenType features to work, but I'm also using the Windows 7 beta. Let me know if you can control the features. To switch the rendering engine to OpenType, go to Format menu -> Document item -> Font tab, and switch the engine to OpenType. Then you will see OpenType feature checkboxes in the Format -> Font dialog.
I recommend Vista for this. You can supposedly get to it in XP if you transplant the Vista version of Uniscribe, which is tedious yet possible.