My suggestion would be BDI. That is the Brain Dead Install of EMC on Linux. This is a distro that will install Linux, the realtime kernel and EMC on your machine.
Xandros is Linux from what I know. It is likely to be possible to install EMC on it. In your case I would not reccomend this at all, building the realtime kernel, the associated librarys and EMC in one shot for somebody with limited Linux expereince would be a job and a half.
Go the BDI route, do realize though that your PC will loose everything on its harddrive as you wll likely want to do a clean install on an older machine like this. As for howto, just follow the instructions at the BDI web site. There are other distros besides BDI that have EMC on it, check out the EMC web site for more info.
Be aware that BDI is Debian based Linux, this is not what I favor. Instead I prefer the Redhat/Fedora based distros. If this was a Linux thread that would create heated discussions, it is not and I hope everyone realizes that DBI is the easy way to get into EMC.
If you really want to learn about CNC and realtime control you can consider building everything from source possibly on your Xandros distro or on another distro. It is a good way to learn about Linux and some of the advanced CNC concepts, just don't expect it to be a quick path to success.
You also need ot consider compatability. Linux has difficulty with non mainstream hardware and EMC just makes things a bit worst. Something to consider, older hardware is generally better but there are gotchas out there.