Hope I'm not late in the game on this one. But I found a tool advertised as able to scan for machines that are either infected or vulnerable to infection from the Nimda worm/virus. http://www.eeye.com/html/Research/Tools/nimda.html

From the website: "The Retina Nimda Scanner is a tool created by eEye Digital Security that is able to scan up to 254 IP addresses at once and determine if any are vulnerable to the "Nimda Worm". If a machine or server is found to be vulnerable to the Nimda Worm, the Retina Nimda Scanner will flag the IP address. " Hope someone finds this useful. I have no affiliation with eEye. -BM