2000: Windows 2000
Screenshot courtesy of GUIdebook.
Windows 2000, the successor to Windows NT 4.0, released in February 2000, was intended for business rather than home use, and it was available in several editions, including multiple server versions. It brought many features of Windows 98 into the NT line, including Internet Explorer and Plug and Play. Windows File Protection, which protected important system files, was introduced along with the Encrypting File System, which improved security by encrypting files automatically, and Active Directory, the enterprise technology used to provide network and domain services.
System requirements for Windows 2000 varied depending on whether the server or desktop version, called Windows 2000 Professional, was being installed. Windows 2000 Professional required at least a 133-MHz Pentium microprocessor (or the equivalent), 32MB of RAM (64MB was recommended) and a 2GB hard disk with 650MB of free space.