Hope this link can help you

http://service1.symantec.com/support/ent-security.nsf/854fa02b4f5013678825731a007d06af/3aa5a06c7bf48bd988257589006cd1e1?OpenDocument

Conficker wont infect a system with all of its Windows patches.

Run the Microsoft Baseline Security Analyzer to find out what patches you are missing.
http://technet.microsoft.com/en-us/security/cc184923.aspx

Update your windows with all the patch, including the one against downadup, and make sure you have the newest virus definitions. I think it this one for downadup http://www.microsoft.com/downloads/details.aspx?familyid=F26D395D-2459-4E40-8C92-3DE1C52C390D&displaylang=en

hi jo,

You said that the virus keeps coming back.

hence make sure that the system in your network has SEP installed with latets defs & also all the system has the microsoft patch installed.

Further information on downadup is available in the .pdf located here: www.symantec.com/content/en/us/enterprise/media/security_response/whitepapers/the_downadup_codex_ed1.pdf 
This detailed write-up comes from our Security Response Team and can help with understanding how downadup works. It also contains great information on how to respond to downadup infections, as well as, response procedures for re-infection.

I think You are missing some Microsoft security patch. Pls ensure KB960714 and KB958644 installed in all pcs which is present in your network.