"This kernel requires an x86-64 CPU, but only detected an i686 CPU" I was trying to install it on a old PC that I acquired which has a pentium ( I think) and 512mb of ram.
why might that message pop up?
Because you've tried to install Lubuntu-amd64, the 64-bit version.
Download the 32-bit version (http://cdimage.ubuntu.com/lubuntu/releases/14.04/release/lubuntu-14.04-desktop-i386.iso.torrent) and install that.
Zoltan is correct. Download the 32bit version and install that.