under eng xp sp3
C:\Documents and Settings\somebody>taskkill /IM firefox.exe
SUCCESS: The process "firefox.exe" with PID 1324 has been terminated.
C:\Documents and Settings\somebody>taskkill /IM firefox.exe
SUCCESS: The process "firefox.exe" with PID 1324 has been terminated.
C:\Documents and Settings\somebody>taskkill /IM firefox.exe
ERROR: The process "firefox.exe" not found.