There's also someone in that thread saying it happens for them with uTorrent and they aren't using Opus.
Yup that is why i am not convinced it is OPUS. Been looking into uTorrent as being the culprit, will post any resolution I find regardless of what app ends up being the cause.
I noticed the other thread you found & replied to:
forum.utorrent.com/viewtopic.php?id=96042
That one also has someone experiencing the problem who doesn't have Opus installed, and he says:
So it sounds like in one of the uTorrent 2.2 updates they changed how they open folders to a method which doesn't work properly if the default folder handler is changed at all, even if it's still set to use Explorer but in a slightly different way.
Why the uTorrent team can't just look at what changed in that version and work out what's causing it, I don't know. 
FWIW, I still have uTorrent 2.0.2 (the update checker seems to be completely broken ) and that works fine in this regard.
I agree. Is okay I have a workaround that prevents the situation. Thanks for the help, it was kind of a squirly thing to track down.
I am having the same problem. I hope we can a solution soon.
The solution is for uTorrent to fix their bug.
Just a quick update: We've worked out what's going on here, at least if our guess about what changed in uTorrent is correct, and have been testing a change that will get rid of the problem.
The change won't be in 10.0.3.0 (the next stable release) but will be in 10.0.3.1 (the beta release that will come soon after it) sometime in the new year.
That's assuming we don't run into any problems with it, but so far it looks good.
FWIW, the issue is a combination of Windows, Opus and uTorrent all doing things that are mysterious to each other, and differences in how undocumented parts of the Windows API have been interpreted. We've worked out what the other parts are expecting Opus to do here, so everything should be good once the change is released.
(I'm still not sure why uTorrent is launching a second window here, but my guess is it has something to do with making it work under WINE/Linux, where the API in question is not implemented at all and it would need to fallback on an alternative method avoid not opening any window at all. Doesn't really matter why, though, so long as the fix we're testing works.)
Just adding this: Firefox 9.0.1 also makes two Listers open. In FF's downloads window, when I right-click a downloaded file and choose Open Containing Folder, two Listers open, same as they do with utorrent.
And there's no way of tweaking this in Windows 7!
The same change, once released, will fix the issue with FF9 as well.
