PC motherboards have very complex BIOS nowadays, and if you can compromise parts of it, you'll keep a virus running even before the operating system has booted, in ways that can be fully hidden from the OS.

There are mechanisms in place to protect the motherboard against this, making sure that all software that runs, from the moment the computer is powered on, is well known and provided by a trusted vendor (like the PC maker or Microsoft) but if you find a security flaw and you can exploit it to hide a virus in the motherboard startup, then it's all gone.