So my startup needs to create intro video that explains what our service do.

I have already wrote the scripts (I used to write screenplays). I try to simplify each scene so that they can be accomplished by just using moving images (similar to dropbox's video). I know they used stop-motion to accomplish that, but I imagine there must be an even easier way to do it all on the computer.

What software would be good for that task? Flash?

Also, voice over syncronization is a different matter, but if you know a good tool for that, that will be helpful too.