This build isn't provided by Valve.
Softpedia has stumbled across a Reddit member who has created a version of Valve's SteamOS platform without the UEFI requirement. The original version, launched last week, supports the new firmware interface, leaving those stuck with the old BIOS unable to use the new Linux-based game-focused operating system.
According to the Reddit listing, grub-efi was replaced by grub-pc to drop the UEFI dependency, and the apt-cdrom repository was rebuilt. The default.preseed file was edited to enable manual partitioning, and the final ISO image was created using grub-mkrescue with the original grub.cfg.
To download the unofficial UEFI-free version of SteamOS, head here. Remember, this version and the official build are in the early beta stage, so keep that in mind when installing and taking the software for a test drive. Inexperienced gamers are suggested to wait a while before downloading Valve's new platform.
PC World has provided its first impressions, reporting that SteamOS requires a Nvidia GPU; those offered by AMD and Intel are not supported at this time. SteamOS is also incapable of existing on a machine with Windows already installed, thus users must have a separate machine or a special drive for Valve's platform. Installation took around 20 minutes, according to the site.
For those who don't want to download a modified OS, there's a UEFI workaround here via Reddit, instructions for installing SteamOS on a virtual machine here, and a way to install Windows on a SteamOS-based machine here.
While you're here, take a look at our guide to building your own Steam Machine here. As previously indicated, you'll need a Nvidia GPU due to the current SteamOS limitations on that front. Other ingredients in our recipe for a home-built Steam Machine include the Azza Z case, an Intel "Haswell" processor like the Core i5-4670K and i7-4770K, the MSI Z87I-Gaming motherboard, and more.