There is another way.
every video after my third had/has this exact problem (too many transitions/effects and it's a crappy program).
If you do not want reduce video quality (which is bad enough in wmm.) and reimporting the source footage doesn't work
you could try this approach.
1.) save as your project file calling it something different 1st_half for example (note: not save movie file).
2.) Try saving the movie file, delete any footage/clips etc after the point where you cannot save the movie file (you may need to save a few times to figure out where this is) it's best to save as highest quality or as recommended
here
3.) once you have a partial file saved, open up your original project file and import the saved 1st_half, replace the footage at the begining up to where you save with the newly imported file and don't forget to mute.
note; you will lose some vis quality by using this method but it is relatively minor, and you could always try cleaning with AVIsynth.
This has also NEVER caused a problem with my sync, whereas saving as lower quality can.
I usually get stuck and have to re-create the second half of the video because I didn't sfollow step one (hehehe poor planning on my part)
I have tried cutting the song in some instance and reattaching the files but this became more trouble than it was worth and I found just pulling the music back onto the timeline worked fine.