Windows 2000

Windows 2000 (codenamed NT 5.0) is an NT-based version of Windows released by Microsoft. It succeeded Windows NT 4.0. It's Professional SKU was replaced by Windows XP, while it's Server SKUs were replaced by Windows Server 2003. Windows 2000 was the last NT-based Windows product before the unification of the Windows NT and DOS-based line. It reached general availability on 2000-02-13. Microsoft ended support for Windows 2000 on 2010-07-13.

Windows 2000 targeted both the high-end consumer market, as well as server and business markets. Multiple SKUs were made for both client and server uses, albeit the user interface remains largely the same. Files from Windows Server 2003 build 2462 show a "Windows 2000 Personal" login banner, which indicates that such edition could also be in planning.

Editions
There are 4 major editions of Windows 2000. They are - Professional, Server, Advanced Server and Datacenter Server. Windows 2000 was intended mainly for use in businesses and Windows ME was more targeted at home users, however, due to Windows ME's negative reception, many home users ended up buying the Professional SKU of Windows 2000 during the time.

Service Packs
There are 4 service packs of Windows 2000. Windows 2000 SP4 is the latest version. Microsoft had originally intended to release a fifth service pack for Windows 2000, but Microsoft canceled this project early in its development, and instead released Update Rollup 1 for SP4, a collection of all the security-related hotfixes and some other significant issues.The Update Rollup does not include all non-security related hotfixes and is not subjected to the same extensive regression testing as a full-service pack. Microsoft states that this update will meet customers' needs better than a whole new service pack, and will still help Windows 2000 customers secure their PCs, reduce support costs, and support existing computer hardware.

Leaking of source code
On or shortly before 2004-02-12, "portions of the Microsoft Windows 2000 and Windows NT 4.0 source code were illegally made available on the Internet." The source of the leak remains unannounced. Microsoft issued the following statement: "Microsoft source code is both copyrighted and protected as a trade secret. As such, it is illegal to post it, make it available to others, download it or use it." Despite the warnings, the archive containing the leaked code spread widely on the file-sharing networks. On 2004-02-16, an exploit "allegedly discovered by an individual studying the leaked source code" for certain versions of Microsoft Internet Explorer was reported.

About Service Pack 5
Microsoft planned to release Service Pack 5, but it has been canceled and Update Rollup for SP4 has been released instead.