Tracking cookie ad dot yieldmanager dot com problem

Hello,

I am using Endpoint Security & Control version 9.7

I ran a complete scan of the system and was UNABLE to find  the tracking cookie " ad dot yieldmanager dot com".

The scan results:  ITEMS DETECED : ZERO    Items Quaratined : ZERO

However, I noticed that the system was bit slow, so I used NORTON PC Checkup to scan the system, a free tool available from Symantec.

It flashed a message suggesting AN INFECTION of my system with  a tracking cookie ' ad dot yieldmanager dot com'.

I am surprised as to why would Sophos miss this ?

Is this particular tracking cookie , harmless .

My browswer settings for IE 9 are set to DELETE all cookies when exited. 

For Firefox 10 I have it set to "NEVER REMEMBER HISTORY" and checked " tell websites I do not want to be tracked".

My questions?

Is this cookie harmful ? If yes, why would Endpoint Security & Control 9.7 miss this ? 

'will appreciate your suggestions.

Thanks,

Dinesh