Call of Duty: MW2; Security Problem on Host – Disconnecting for Safety

Hi Friends,
Have you ever played Call of Duty: MW2 in multi player mode? I tried several times but an error message prevents me. The message contains "Security Problem on Host – Disconnecting for Safety." I don't know why such problem occurs. I have double checked my internet line. I am using DSL cable line and use Windows firewall. Is this any kind of virus related problem or any network problem?
ERROR
Security Problem on Host – Disconnecting for Safety.
Thanks
Melisa Jackson
