...and don't try to install Ubuntu and then LinuxCNC. LinuxCNC includes a custom-built Linux kernel that has been specially crafted to improve real-time operation. Download the kit from the LinuxCNC site, burn a DVD or write to a memory stick (the site gives instructions) and boot/install from that. That gives you all you need - it works pretty well. I went through the process a few weeks back when rebuilding a PC.