Conficker

Conficker (also known as Downup, Downandup, Conflicker, and Kido) is a computer worm that surfaced November 21, 2008, with Conficker.A and targets the Microsoft Windows operating system. The worm exploits a known vulnerability (MS08-067) in the Windows Server service used by Windows 2000, Windows XP, Windows Vista, Windows Server 2003, Windows Server 2008, and Windows 7 Beta.

The Conficker worm spreads itself primarily through a buffer overflow vulnerability in the Server Service on Windows computers. The worm uses a specially crafted RPC request to execute code on the target computer.

When executed on a computer, Conficker disables a number of system services such as Windows Automatic Update, Windows Security Center, Windows Defender and Windows Error Reporting. It receives further instructions by connecting to a server or peer and receiving a binary update. The instructions it receives may include to propagate, gather personal information and to download and install additional malware onto the victim's computer. The worm also attaches itself to certain Windows processes such as svchost.exe, explorer.exe and services.exe.

Source
Conficker Working Group.