I don't know that it's a PLUG-IN really... and it depends on the format that you're outputting to in WMM in order to achieve it.  Let me load it up real quick...

...

Okay yeah, after reminding myself how limited WMM is, I can see there's just no real easy way to use it to give the video a widescreen effect.

However...

...there's a free program called 'Virtual Dub' ... it will only save in .AVI format(either a HUUUUUUGE AVI file, or if you have it, a DIVX-encoded AVI that's much smaller), but it has a very simple 'resize' video filter(I wouldn't really call it a filter, but whatev), and you can use that to resize it in one of two ways.  First, you could just resize the video to a widescreen format, but then it wouldn't have the black bars of course.  The other is to resize the video, but resize the frame below that in that particular filter's options.  It's hard to describe without being able to show it, but basically you're resizing the video, but making sure the frame extends in order to get that black-bar effect.


The other video editor that I bought from the store(which isn't sold or supported any more unfortunately), it's able to do it fairly easily...but I can't really tell ya how to do with with that since you probably can't get it anyway...