Backdoor.Trojan can't be removed/cleaned

one of the users in the company domain got Backdoor.Trojan and can't be cleaned, quarantined. I tried to restart in Safe mode and full scan, it threw the same msg below.

 

BTW, we are running Symantec antivirus corporate edition 10.1.5.5000 in the server, symantec client security at client. all defition is up to date. Please help us!!Thanks!

 

Event:security risk found!

risk:Backdoor.trojan

file:c:\windows\system32\userinit.exe

location:unknow storage

computer:computer name

User:domain\user1

action taken:clean failed:Quarantine failed

date found:28,jan, 2009. time.....