My nephew recently got one of those plastic engine models for Christmas.
http://www.discoverthis.com/motorworks.html
I was helping him put it together and found it very educational. Anyway, I was wondering… is there anything like this for learning how to build a PC? I know their are lots of pictoral guides on the internet, but is there actually a PC model that you can buy that has a little fake tower with little fake cards and whatnot that you can install?
For what such a toy would cost, go buy a 486 or Pentium, gov’t surplus, and a $20 monitor, and completely disassemble it. Bring him the parts packed in anti-static bags, and all the tools he’ll need (thermal paste for the CPU, philips-head, case screws). You can burn a Knoppix CD or just put Win98 (ugh) or Win95 (ugh-er) on it, and then he’ll have a real working computer to play with.
Install a few games on it like Typing Tutor or some great abandonware from Underdogs, and let him enjoy the fruits of his labor!
Well, the OP didn’t say the plastic model was for his nephew. 
I’m with Jurph. Building a real engine is complex, messy, requires a lot of special tools, large space to work in, etc. So building a plastic model demonstrates a lot of the principles without the fuss. But for electronics, with nearly no moving parts, there’s not much benefit to having a plastic model PC. Once it’s built, what can you do with it? It wouldn’t be that much harder to build a real PC.
Note that an engine is a mechanical device and that most of the the principles of operation can be demonstrated with a plastic model. The same cannot be said of a plastic model of a computer and I think that is why no one has probably bothered to build one.
The PC, not the monitor. Monitors can kill you.
To clarify,
Monitors can hold huge charges inside them, even after they’ve been turned off and unplugged for a long time. They’re fine to carry and handle, and there’s not much evidence that using them is particularly bad, but if you take it apart, you could get a huge shock, easily enough to kill you.
On the other hand, if your PC is unplugged (not just turned off, but physically unconnected to any power source), then there’s little damage you can do to yourself with casual taking apart. You can pretty easily do serious damage to some pieces (for instance, by handling memory enough to build up static), but it’s hard to hurt yourself.
BUT DON"T EVER TAKE APART MONITORS!! EVEN UNPLUGGED!!
Right! YES! Do what they said. Do not take apart the monitor or let the tykes do it. Bad idea and a half. I withdraw my poorly placed pronoun.