Todays most commonly used operating system is windows Xp. Its easy to install and use by the home users.
What Is The Most Commonly Used Operating System In The Market Today?
The most commonly used OS is Windows Vista, as XP is unable to be preloaded into non-netbooks.
I'm pretty sure it's Ubuntu, just because it's free. Think about it. All the poorer people in Africa (well, at least the ones able to at least afford a monitor) use it.