The same company also have a Djvu Shell Extension pack which includes a preview handler that Opus will use:
http://dev.caminova.jp/beta/djvu-wic/
The preview handler seems a bit nicer as you don't have to tell Opus to use it (Opus will use it automatically if it's installed) and it scrolls documents in a better way. That said, the Djvu ActiveX control has some features that the preview handler doesn't (e.g. a ruler to measure distances, options to rotate pages, etc.)