Bootkit

A type of rootkit that infects the Master Boot Record or Volume Boot Record (VBR) on a hard disk drive in order to ensure that its code will be run each time the computer boots.

Modern bootkits (such as Mebroot, TDL4 - Olmarik, or Rovnix) also use this technique to bypass certain Operating System security mechanisms by loading the malicious bootkit code even before the OS is fully initialized.