I'm not sure what you mean by the "free trial" of Movie Maker. There is no "free trial" version of Movie Maker. The program provided by Microsoft is part of the free "Windows Essentials" download and there is no "trial" version, free or otherwise.
The first thing I would do is make sure I'm running the latest official version of Movie Maker. It will be able to read older project files as well as newer project files. But just to be sure, make a back-up copy of your project (.wlmp) files. Some of the older versions of Movie Maker were more prone to crashing, so I'd recommend the latest regardless.
Where to get a Safe Download of Movie Maker
The next thing I would do is immediately try a save of your project as a movie. I'd hate for you to lose your in-progress work and not even have a single save of the movie so far. Remember the *project* is not actually a movie. You always want to try a save as a movie as you go along to ensure you don't encounter problems toward the end of the project.
Another thing I would do is to always save your project with a *different* name each time. That way if one of the project files is lost or corrupted in a crash, you can go back to the one before. I usually save the project with numbers like "MyMovie01" "MyMovie02", etc. Or perhaps the date.
Can you provide some details on the number of clips in your project, the length of your movie, the types of video clips you are using, etc.?