You can upgrade to a better video program, and record the timer and then have the two videos simultaneously.
...buttttt chances are you didnt want to hear that. You probably just want to use the default program. I dont really know, i use sony vegas for my videos.
I would use a PC capturing software that can capture what s going on your comptuerscreen in video format.
Problem is, you cannot use Picture-in-Picture with Windows Movie Maker as far as I know so yeah.... you need a better video editing program.
I tried using Adobe Premiere but it's clumsy, too advanced and just overcomplicated.