On the OPML Support mailing list a few days ago, Andrew Grumet posted the reason why the OPML Editor doesn't work with IE7: 
It's the htmlcontrol that renders Harry S and the status message. If you edit the source db to prevent its invocation, the app launches and runs fine (assuming you don't need anything that was exposed in the html). My guess is that the IE6 to IE7 upgrade broke it somehow.

Presumably, one could open the source in Frontier, edit it and open it up. 
According to the folks on the frontierkernel developers list, it doesn't seem that there is a reason why Frontier (and, by extension, the OPML Editor) won't run on Vista - except that the rendering of 'wp-text' is done using a library that has changed. 

The idea is simple. I release, you comment, I read the comments, change the ontology and then re-release. 