How many of you would be able to make a working, programable computer? (i.e. not from pre-built parts)
Please explain this question. What counts as a prebuilt part? Certainly a motherboard or graphics card, but what about a RAM module? A processor? A microcontroller? EEPROM chips? Logic (ie AND, OR NOT, 555 timers etc) chips? Transistors? Resistors? Capacitors?
Where do you draw the line?
If you really do mean NO pre-built parts, then I don't think theres very many people who could do it. You mention the £15 computer that guy built, but he uses plenty of prebuilt things: ARM processor, USB controller, EEPROM, SD card reader, oscillators, resistors, capacitors, wiring (I'm taking an educated guess as to what parts he used - I did not look it up). If you have to build everything from scratch, what about purified silicon?
It of course is possible to do this and thats how we eventually got to where we are today, but there is a LOT of detail and many many components in a modern computer. I doubt any one person could build one from scratch from nothing.
If I can use basic semiconductor components and basic logic and memory ICs, I could possibly build you a very very very simple computer... but it wouldn't really resemble a modern computer and not be nearly as capable.