The US Dept. of Homeland Security released "the most comprehensive [tool] available for enterprises like federal and state government and private sector networks to determine the extent to which their systems are infected by Conficker worm." DHS is distributing this detection tool only through its secured channels. Specifically, government sources may acquire the tool through the Government Forum of Incident Response and Security Teams (GFIRST) portal; and private sector partners may contact their designated Information Sharing and Analysis Center (ISAC).
US-CERT encourages users to prevent a Conficker/Downadup infection by ensuring all systems have the MS08-067 patch, disabling AutoRun functionality (see http://www.us-cert.gov/cas/techalerts/TA09-020A.html)