On uninformed.org you can find a whitepaper which describes several ways to get your code covertly executed in the Windows kernel.
This paper presents a detailed catalog of techniques that can be used to create local kernel-mode backdoors on Windows. These techniques include function trampolines, descriptor table hooks, model-specific register hooks, page table modifications, as well as others that have not previously been described. The majority of these techniques have been publicly known far in advance of this paper. However, at the time of this writing, there appears to be no detailed single point of reference for many of them. The intention of this paper is to provide a solid understanding on the subject of local kernel-mode backdoors. This understanding is necessary in order to encourage the thoughtful discussion of potential countermeasures and perceived advancements. In the vein of countermeasures, some additional thoughts are given to the common misconception that PatchGuard, in its current design, can be used to prevent kernel-mode rootkits.