What is conficker worm?

The Conficker worm is a computer worm that can infect your computer and spread itself to other computers across a network automatically, without human interaction.

What does the Conficker worm do?

It might spread through file sharing and via removable drives, such as USB drives (also known as thumb drives). The worm adds a file to the removable drive so that when the drive is used, the AutoPlay dialog box will show one additional option.
The Conficker worm can also disable important services on your computer.
In the screenshot of the Autoplay dialog box below, the option Open folder to view files — Publisher not specified was added by the worm. The highlighted option — Open folder to view files — using Windows Explorer is the option that Windows provides and the option you should use.
If you select the first option, the worm executes and can begin to spread itself to other computers.

How does it work?

How to prevent that worm

If your computer is infected with the Conficker worm, you may be unable to download certain virus protection security products, such as the Microsoft Malicious Software Removal Tool or you may be unable to access certain websites, such as Microsoft Update. If you can't access those tools, try using the Microsoft Safety Scanner for virus removal.

Source:

http://www.microsoft.com/security/pc-security/conficker.aspx#ETB