@cybmole - I am amazed anyone is still using "Open epub in editor" plugin given that Open With complete supercedes it. However neither will be the source of the "problem".
There is an illuminating article here:
http://windowsteamblog.com/windows/b...ts-part-2.aspx
From reading that it should just be as simple as registering a file handler on Windows. I am not at home at the moment to try it but it should be a simple addition to the Sigil windows installer.