I really hate resurrecting old threads but I haven't been able to fix this. Each of the solutions listed involves opening video files outside of LW, and I have no idea how to do this, as I'm not a very technical person. If someone could post a step-by-step way of solving my problem, they will have my eternal gratitude.

The easy way is to go to your LW library, click on Saved Files then click on Explore. That will open your saved folder (outside of LW). Now you can change your default player by right clicking on a file > open with > choose program > click on WMP > tick the box saying "always use this program to open files of this type" > ok.