My approach would be to pick up a PC case at your local dump. (Don't chuck 'em aware where you live ? They do in Spain).
Gut it completely and pop along to Maplins, pick up a power supply, drives, motherboard and processor and a couple of fans, stick them all in the case (it is very easy if you had a Meccano set as a kid !) buy the software of your choice (WIN7 would be my choice, if you don't want to pay then Linux) load it and off you go.
You might not want to build one but this way you'll know what's in the case and be able to swap RAM and drives when necessary out of your spares box(es).
Last edited by OFSO; 3rd June 2014 at 14:36.