Can Steam OS/Steambox run Windows games?

The functionality being referenced when talking about playing Windows games on Steam OS (which is a Linux derivative) is not currently present in the current releases of Steam OS. The functionality in question is actually licensed from Nvidia and is the same streaming technology used in the Nvidia Shield, and will appear in Steam OS closer to its official release.

This technology doesn't allow you to "play" Windows games as much as it allows the heavy lifting to be performed on a Windows machine on your local network and then streamed to the box running Steam OS.

On your official linked page there is a explanation: It's not playing on our Steam machine, as it seems, it's more a streaming technology.

In-home Streaming

You can play all your Windows and Mac games on your SteamOS machine, too. Just turn on your existing computer and run Steam as you always have - then your SteamOS machine can stream those games over your home network straight to your TV!

So naturally playing Windows games on Steam may not be possible. But as you can modify Linux for your likes, you can try WINE, PlayOnLinux or similar software.