Hi Zafar,
Thanks for the writing your query in Symantec community.
In this case active threat might be present on the System itself.
Scan the full system in safe mode, if required use Symantec power eraser tool to scan the system.
Here is the location of the Symantec Endpoint Protection Support Tool:
http://www.symantec.com/business/support/index?page=content&id=TECH105414
Use Power Eraser to detect threat and remove them
http://www.symantec.com/theme.jsp?themeid=spe-user-guide
Make sure all three SEP features are installed, Windows is updated with latest patches & Service packs.
Best practice should be followed : http://www.symantec.com/docs/TECH105236
Also run Sep support tool (SST) on the infected machine & Submit files that are malicious. We suggest that you submit any such files to Symantec Security Response. For instructions on how to do this using Scan and Deliver, read Submit Virus Samples.