problem occured when I update virus definition of my Symantec Sever

Dear friends,

I install a Symantec Antivirus Server on MS Server2003. When I live update its virus definition through Internet, the server is automatic shutdown by NT Authority/System after few minutes.
Kindly send me the solution how can I update virus definition my parent Symantec antivirus server through live update method?

Best regard,

Shahzad