The prompt that came up when selecting the hard drive is not advising that the old files will be erased, but that they will be moved to a folder on the hard drive called windows.old. the windows.old folder is useful in the event that you may want to restore your old OS, or you can sift through it and pick out your personal files. Windows? 7 will not format your drive by default. Vista also did the same thing, I worked on the official vista support team at launch, and I will say that feature is brilliant for sides, as a user, there's not accidentally erasing data, and as a tech, there's no accidentally walking a user through erasing data."