...of course. But my friend's mother HAD been using Windows for several years, surfing, emailing, even doing some simple Photoshop work...
And at this point, the similarities between desktops are greater than their differences: the GUI hasn't changed much since it was codified in the 80s, right?
You've got icons, menus, a desktop, a means of browsing the filesystem, some gadgets widgets and doo-hickies...
You click, double-click, right-click, shift-click, control/command-click, etc.
Touch interfaces are the first really new wrinkle in 25 years.
I think the biggest thing that determines your success with an OS is your attitude and the pre-conceptions you have (give to you by others.)