Sara,
You can mix mpegs together easily using Movie Maker .
Assuming that you have Windows XP:
- just download MM 2.1 from Microsoft (free download).
- import the clips
- Drop the clips into the movie timeline
The interface is very intuitive and the help is pretty good. There are lots of sites online to get you going and help you past some of the speed-bumps. The best site I've seen is here:
http://www.eicsoftware.com/PapaJohn/MM2/MM2-Online-Tutorials.html
I even bought the guy's book -- which is pretty basic -- mostly just to toss him a few bucks for spreading the word. In my view, Movie Maker is one of the better apps that Microsoft has ever produced.
those contemplating a "witty" reply -- please spare us:naughty
Good luck and if you have any detailed questions, like how to create your own transition and special effect filters, do chroma keys, tweak the color balance for underwater films etc., I'd be happy to share what I know.