Open Source E-Commerce Website Builder Installation

I am trying to launch an e-commerce website and have tried numerous different sites including, among others, osCommerce and C.R.E. Loaded. On downloading each one I am invariably faced with masses of php and css files that open as plain text documents, and after trawling through it all to find the installer, find it links to a webpage filled with infinitely complex and confusing instructions. I'm sure there's something obvious that I am missing but can anybody tell me what it is?

It doesn't sound like you are missing anything.. you just need to follow the instructions.. that's how you usually install web applications...

To be honesy I have to say I didn't understand which software did you choose to use. If you need shopping cart Zen is fine. There are some paid solution in "under $20/month" range that also work fine.