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On this day in 1991, Linus Torvalds announced he was working on what would become Linux
(www.xda-developers.com)
This is a most excellent place for technology news and articles.
I don't think being able to run programs designed and compiled for windows is a requirement to be considered a usable os. For example, you can not run safari on windows. Does this mean windows doesn't count as a usable os?
I think the definition of useable should be that software exists that can do the kind of things you want to do on your computer. In that sense, Linux is perfecty useable on the desktop, at least for people who have similar computing requirements to me.