w32.downadup.B virus

My Antivirus (Symentec) detects this virus and successfully quarantines it. After sometimes it appears again and gives a msg like “quarantine failed”. The effected file is winnt/system32/x. Sometimes it shows that the effected file locates in Temporary internet File folder. But actually no file locates there. And it has spread whole LAN.

I tried with Avast as well. no result. Can anybody pls suggest me anything?

Thanks
Anjan