NEED HELP in DISABLING Anti-Virus Service-Meterpreter

I am stuck at post exploitation scenario.

using bugged pdf i am successfully getting a meterpreter prompt. i can successfully close the firewall and windows balloon on the target system(Win Xp Sp 3) using the meterpreter

But the problem is with antivirus.. Killav.rb command is not potent .so i am trying to disable the antivirus service using following command in shell.

sc config start= disabled

I am getting error as access denied, I have administrative rights, i am sure about that..

what i have found is that all latest AV be it avast, avg etc are coming with

SELF PROTECTION MODULE . and i think without disabling the module Anivirus cant be killed on the target system..

Can any one tell me how to disable the MODULE, once its diabled i can succesfully disable the AV..