HELP!!! trojan.zeroaccess.b

Norton Anti Virus keeps popping up telling me it has blocked or resolved a threat from trojan.zeroaccess.b  - it also asks me to reboot my computer.  This message pops up repeatedly and wont go away.  I have run a virus scan, I have run the norton  removal tool, I have run norton power eraser - they all say that the threat does not exist, yet i still get pop up messages from norton saying it is blocking auto-protect blocked the threat and then details show that the threat is being resolved.  HELP clearly I have a virus and cant get rid of it!!!!!