In-depth Conficker/DownadupReference
Here is a resource by Symantec explaining Conficker. They include a nice piece by 60 Minutes explaining Conficker in non-technical terms. We've included additional reference links below.
Quick Machine Test for Conficker Infection
The Conficker Working Group has created a simple web page to help you determine if your machine is infected. Since Conficker blocks access to various security sites, this simple attempt to load logos can quickly tell you if you are infected. If some of the logos do not load, you might be infected. Click here to visit their quick and easy test.
Quick Scan for Your Network for Conficker Infections or Vulnerabilities
If you want to quickly scan your entire network for Conficker infections or Conficker vulnerabilities, use nmap. I found a great example of how to do this at OS X Daily. As noted in the original article, be sure to change the IP address range to match your network, and the relevant code is here:
nmap -PN -T4 -p139,445 -n -v --script=smb-check-vulns --script-args safe=1 192.168.0.1-254
The scan will tell you if any Windows machines are actually infected or lack the patch to prevent infection and are vulnerable.
Other Conficker/Downadup References