In a post-apocalyptic world would you even have access to electricity? Assuming nobody can operate electrical plants or pipe oil your machines would be dependant on natural energy sources or powered by pre-apocalypse batteries. It would have to be a tough machine that is extremely power efficient. Then its just a matter of having a Unix like OS that could run on such hardware that comes with a text editor and all the essential programs. The network infastructure for modern telecomms and the internet should still be intact even if its in ruins. So there should be no problem in creating computer networks if there are enough people willing to start one up.