A couple of insecurity researchers are about to release a research paper and exploit code for an SMM (System Management Mode) rootkit, that uses an Intel CPU caching vulnerability to install.
Joanna Rutkowska and Loic Duflot claim this is the scariest, stealthiest, and most dangerous rootkit ever. It installs and hides in the SMM space, which is off limits to any OS, thus making it impervious to security measures.
What's worse the SSM has been around since the time of Intel's 386 CPUs, when the Berlin Wall was not a tourist attraction, but rather a place you visited if you wanted to put an end to your misery. The researchers claim Intel has been aware of the issue since 2005, but has done nothing to fix it.
There is no way to combat the threat, or even know if someone is already messing with your PC. So, relax, we can't do anything about it, or even know if it's there.