This chapter explains and compares internal structures of Windows3.1 and Windows95. Good references to Windows3.1 has not yet been obtained, however, [8] could be an information source for Windows3.1 since it described Windows95 in contrast with Windows3.1. Some of the informations about MS-DOS were obtained from [10]. Section 8 of [5] was also the source of general information of MS-DOS. Due to lack of information source that exhaustively describes both Windows3.1 and Windows95, the degree of detail differs from section to section. Again the section of networking (2.6) is less theoretical and technical than other sections.