My first suggestion is to take the computer to BestBuy and ask them to put in more RAM if possible. (Or if you're the adventurous type, do it yourself.)
If you can get 512MB of RAM, you could try Google SketchUp. (
sketchup.google.com/) It is totally different than AutoDesk Inventor (my only other CAD experience), but it is quite intuitive and great for quick prototyping...
Hope this helps!