You could go with Game Maker 8 (25$ for the full version that includes particle systems) for an easy way to produce a demo or even the full game. It supports everything you need and, best of all, drag-and-drop development for non-coders. I have some step-by-step tutorials on my website for getting started quickly with a Pong and Breakout styled game (will show you all the basics of the engine).
You can find tutorials on making a side scroller on other websites, including the Yoyogames official tutorials. The official Yoyogames.com forum is also a great resource and place to ask for more specific help.
In my opinion the biggest downside of Game Maker is that it currently is Windows-only. A Mac version is in the works and almost complete but it has been put on hold due to the lead programmer leaving the project. There are rumors of some form of hand held release due to this.
But in all Game Maker is a really nice tool that is easy to learn and has a low cost of purchase. You can have a nice demo running within a day if you run through a few tutorials first.