Exerpt from Malicious Mobile Code, Virus Protection for windows.
Microsoft Windows started out strictly as a shell menu to hide the roughness of DOS, but it is slowly lessening its rreliance. Microsoft has two core Windows platforms: 9x and NT. Although they look alike, they are significantly different under the hood...
...All windows platforms go through a complex booting routine, different among platforms, but with some commonalities. First every Windows boot begins with a text or non-GUI boot portion before Windows really begins to boot up. It is during this process that low level hardware details are worked out, and enough system files are piled into memory to allow the larger Windows kernel to load. As windows starts up, dozens of files are loaded into memory...
PC Passes POST Tests
Launches Win Boot Process
Text/non-GUI boot portion initiates
Pre-OS drivers and boot files loaded
Windows boot code initialised and loaded
Registry Checked and Consulted
Autostart Programs, Processes, and services started.