W2.Downadup infection...Please help!

Hello every one, My network with about 100 computers having a symantec AV Manager is infected with W2.downadup. Users usually get pop-ups regularly indicating that an admin account (which has logged on to the SEPM server at one time or the other) was trying to access their computer but that the access has been blocked.
This is so annoying as the pop-up occur on a regular basis through out the network.
I have used several removals tool: from Symatec, Bitdefender, Norton,and other but non seem to be working.
I have also ensured that SEP Management server has all the latest patches and it's even running server 2003 R2.

Please help