it's called clustering, it requires Linux, meaning no more Windows
and a special program such as Open MOSIX or ROCKS
you have to connect every PC via network, and then cluster it
it's a semi-complicated process
and only yields advantages for doing large number crunches, not the kind of speed your thinking of
and because it'd be Linux (text-line most likely, as it'd probably disable GUI by default to give you the most rescources) you'd have almost no software support compared to Windows
I have heard people talk about connecting two or more PC's at their own house to increase processing power. Is this really possible? If so, how hard is it and what do you need to do it (i.e. software, cables, etc.) and does it really increase performance that much?