Nothing more than an Operating System, that runs in Protected Mode and supports Multi-tasking and Multi-threading, with a nice GUI (sort of) + some basic drivers and Apps....
NOTE: It's out-of-date, the new version will be released soon.
DeReIn is a simple windows command-line application that protects Portable Executable (PE) file content. The contents of the PE file are encrypted and added to a loader that carries out decryption and execution at runtime.