laptop shutdown when cmd launch/open

My laptop running in WinXP SP2 got infected by possible trojan. When cmd prompt is launch, either by Start->Run or via the cmd prompt in the program menu. I have scanned my laptop with different AV such as symantec, AVG and different spyware such as ad-aware 7, spy-bots and AVG antispy and rootkit but it found nothing. I am still encountering the issue. I tried to replace the cmd.exe with one from other PC but still not resolved. Any idea or assistance to solve the issue is greatly appreciated. Thanks.