The 10061 build of the Windows 10 Technical Preview for PCs has found its way on Internet pirate sites, thanks to someone grabbing it from Microsoft's Windows Update link. However, the company has put in some changes in the build's encryption keys that don't allow it to be installed.
The leak, as reported by Neowin, is using a different RSA key for this 10061 build compared to older preview versions of Windows 10. While it's possible that someone might be able to break it, it does look like the company is trying to keep leaks like this from being usable by a wide audience from now on.
The current public build for the Windows 10 Technical Preview is 10049, which was released over three weeks ago to Windows Insider members. It's possible that Microsoft is waiting until BUILD 2015 starts on April 29 to officially launch a new version of the OS to the public.