Valve has dropped support for the operating system, but an unofficial workaround is available.
Some PC users still run the legacy Windows 7. Many vendors have ceased supporting it, Valve included.
Still, gamers can keep using Steam on older systems thanks to a tech enthusiast known as Eazy Black. The developer has backported a beta build of the Steam client that runs on both Windows 7 and Windows 8.
There is one caveat: you must have Windows 7 Service Pack 1 (KB976932) and either KB3080149 or KB4474419 installed, as Steam requires SHA-2 digital signature support to function.
Source: iXBT.games