Windows XP is an operating system made by Microsoft. It is classified as part of the Windows NT series. Windows XP was a popular operating system, but it was often criticized as insecure, as, because of the NT architecture, many viruses for Windows 2000 also ran on Windows XP. Internet Explorer 6 in the original XP was also marked as insecure, and it was basically a colored version of Internet Explorer 5.5, with minor improvements.