MBR Rootkit, A New Breed of Malware - F-Secure Blog

Yup, Master Boot Record rootkits. Run screaming for the hills...

This new Windows MBR rootkit launches itself very early during the Windows startup process without requiring any registry or file modifications. In fact, it is quite surprising that it's possible to write to the MBR from within Windows to begin with.

The MBR rootkit — known as "Mebroot" — is very advanced and probably the stealthiest malware we have seen so far. It keeps the amount of system modifications to a minimum and is very challenging to detect from within the infected system.
Hit the above link to read up on its "stealth features"...