Microsoft Windows 10 has been hinted at a possible release date situated during mid year of 2015, sometime during the summer.
Microsoft has hinted that users of Microsoft Windows 7, 8, and 8.1 will be able to upgrade to Microsoft Windows 10 free of charge, during the first year of release.
Those interested in beta testing or getting more in depth previews of the software can find more information at the Microsoft Windows Insider Program website, troubleshooting the new software while collaborating with developers from around the world.