Only Windows 7 SP1 and Windows 8.1 Update will be able upgrade to Windows 10 through the Windows Update service. Users with older editions will have to install the upgrade from a USB drive or DVD media, loaded with a disk image downloaded from Microsoft’s servers. It will not be possible to upgrade retired versions like Windows XP; Windows Vista will also not be upgradeable.
Windows 10 will use less storage space on your computer. Microsoft revealed information about a new compression algorithm for compressing system files that saves ~1.5 GB of storage for 32-bit and ~2.6 GB of storage for 64-bit Windows.
Also, a new build of Windows 10, 10049, is available at this time. With this release comes the new, awaited Project Spartan web browser.
You can download the Windows 10 Technical Preview Build 10049 here.