Nessus plugin #36036 performs a network based check for Windows computers infected with a variant of the Conficker virus. The scan does not need credentials, but does require ports 445 or 139 to be open between the Nessus scanner and your scanned systems. The plugin is based on research from the University of Bonn in Germany.
Conficker exploits Windows systems vulnerable to MS08-067. Tenable has worked with many organizations to help them perform both un-credentialed network scans and credentialed patch audits with Nessus to find systems that are still vulnerable. We wrote a blog about our typical experiences working with customers performing these scans.