The Conficker/Downadup worm was first tracked in November 2008 (Win32/Conficker.A) and has infected thousands of business networks and millions of PCs since that time. Conficker uses advanced malware techniques to make it difficult to protect computers. Conficker primarily prevents users from installing Windows and antivirus software updates. The worm also has the capability to launch a second stage attack that downloads code that has the possibility to be devastating, such as rogue anti-virussoftware. In 2009, a variant of the worm Conficker.C was also commonly called the April Fools’ computer worm.
See also Rogue Anti-Virus Software Explained in the Did You Know…? section of Webopedia.