The ever most widely used and important operating system was Windows 98. Its predecessor is Windows 95, which is a direct successor to Microsoft's prior standalone operating systems MS-DOS and Windows 3.1. As a major milestone of Microsoft's operating system, many of the classic features and designs that Windows 95 first introduced, such as the Start Menu, My Computer, as well as the classic card game - FreeCell, are still in use today.

Windows XP SP4 Unofficial, as its name says, it's not a Service Pack released by Microsoft Official site, but it is so popular that XP's loyal supporters call it SP4 of Windows XP after the official final one - SP3.