Post-apocalyptic world

In December 2012 it finally happened: nuclear war, global famine, epidemics, unprecendented natural disasters, fuel shortages, global warming and a zombie apocalypse heralding the arrival of the antichrist. After decades of war human civilisation was completely destroyed.

A century later the surviving population has begun to reorganise. What sort of society would they live in? Would it be closer to the Wild West, Stone Age, Middle Ages, or Highland clans? What technology would they use? Would they be able to reverse-engineer scavenged machinery? How much conflict would there be? Would there be leaders and if so how would they gain power (through violence, popular consent or familial inheritance)?

I think the time span your talking about is too short.  100 years??  If this civilisation collapsed, it would take longer than that for folks to begin to reorganise. IMO it would take 100 years for the collapse to fully occurr.

With global war, fuel shortages, epidemics and famine i'd put global collapse at 20 years or less, especially if the world governments take a direct hit during a time of east-west tension (terrorist attacks on the UN headquarters, Kremlin, Pentagon and Chinese Great Hall of the People coinciding with widespread popular rebellions?)