If your computer needs an upgrade to run Prime (or Mathcad 15) it must be an extremely old computer. What makes you think you need an upgrade?
Sorry, but there is no hobbyist version. There is a nice piece of software called SMath Studio that is similar to Mathcad, although not nearly as powerful, and completely free. You can get it here: http://en.smath.info/ Maybe it will do what you need.