This is done with the option /outputfile
Certify.exe /DBServer="wsxxxx02" /DBName="Vxxxx02" /title="Regression test 2010" /stepdelay=0 /Process="a" /screencapturemode=CaptureDeskTop /capturescreenon=CaptureNone /Project="Test" /logstepmode=LogAllSteps /outputfile="\\yyyy\xxx\zzzz\abc.xml" /dbuser="%1" /dbpassword="%2" /user="%3" /password="%4"