You did not download any new OS. All you did was delete a bundled app like NFB, Slacker, etc. that came with the OS. Then the software update prompted you to re-download it. Go ahead, check your apps. It's there again isn't it?
I don't think there's a way around it yet. I would love to be able to 'hide' the icon, then you wouldn't need to actually delete it, thus avoiding the Software Update prompt.