Hi Srikanth_Subra,
Here is a document with information about how to combat W32.Downadup.B:
Simple steps to protect yourself from the Conficker Worm
http://www.symantec.com/docs/TECH93179
This threat is very good at sprading / re-infecting computers, if certain precautions are not taken. Ensure that all recommendations (password protection, patcehs, IPS, etc) are in place.
Hope this helps!
Mick