The first computer that I wrote a program for, in 1963 as an undergraduate student, was already old-fashioned at that time. It was a valve-based computer called Utecom at the University of New South Wales (“Utecom” was short for “University of Technology Computer”, and University of Technology was the former name of UNSW.
Utecom occupied a very large room, and broke down regularly as one of its valves failed. You changed the “operating system” (for want of a better word) by removing and replaced boards wired up different ways. The programming language was called George, and was a reverse-Polish language very similar to that used on HP calculators. In fact Utecom was very similar to, and about as powerful, as the sort of scientific calculator that you buy for about $20: all you’d need to add to that would be a punched-paper-tape reader and writer, and you’d have the whole capability with today’s technology.
I imagine that to emulate a typical modern-day laptop you’d need to fill every building on a large university campus with the processing units. They’d also be the fastest way to emulate RAM, and I don’t know what you could use for a hard drive except more valves: anything else would be intolerably slow. You’d need to build in redundancy, since otherwise the system would fail each time a valve failed, and that would add more valves and some sophistication into the operating system.