"CA (formerly Computer Associates) has published an extensive guide to Conficker.C, which includes information on its attack vectors, behavioral analysis, and how to tell if the "C" variant of Conficker is running on your system. This last part could pose a challenge—unlike previous versions, C adopts what DeBolt refers to as a "defensive stance" and throws up a number of roadblocks, all of which are aimed at hindering user detection of the worm.
The security industry was collectively able to put the brakes on Conficker.B’s expansion when they managed to reverse-engineer the virus and determine which domains it would attempt to register and dial home to on particular dates. With Conficker.A and B, the worm chose to contact 32 addresses out of a possible 250 on any given attempt. With their algorithm broken, the malware authors went a step beyond updating their randomization/selection code—they also vastly increased both the number of domains the worm could generate as well as the number it will randomly select. Conficker.C will select 500 domains out of a randomized pool of 50,000 instead of the previous 32/250." – Content courtesy of Conficker.C primed for April Fool’s activation – Ars Technica