Windows 10 hasn’t available yet, and it apparently will be launched in the Autumn. In the mean time you can install the preview version, for free. It is a beta for sure, addressed for developers actually. It means we wouldn’t recommend that for anyone attempts to use Windows 10 for their main PC. Not just yet, anyway. Still there is the good news: when Windows 10 launches it will be a free upgrade for existing Windows 7, Windows 8 and Windows Phone users. At least for the first year, anyway. Means, it is NOT absolutely free. You need to have the original Windows 7, 8, or windows phone installed on your machine in order to get 1st year free upgrade to windows 10.
So if you are a Windows Vista or -XP user, you may want to think about upgrading now. It seems that you’re not eligible for this free upgrade program.
Another good news, Microsoft will also make the software downloadable as an ISO disc image, for those who want to keep it on DVD or a USB memory key.