Read though the evolution of operating system until nowadays and highlight seven great achievements (golden milestones) in this evolution. Then choose the most important three out of these seven, then choose the greatest two big achievements in the evolution of OS in the last decade. Reason each of your choices in a professional technical writing.
Your starting point might be at:
Timeline of operating systems: http://en.wikipedia.org/wiki/Timeline_of_operating_systems
History of operating systems: http://en.wikipedia.org/wiki/History_of_operating_systems
Google search innovation of operating systems timeline