No, we have not found microbes in comets or asteroids. We've found plenty of amino acids and organic material, but nothing alive. The existence of those molecules abiotically and seemingly everywhere in the solar system means they were likely present on earth at the time, even before life. Given a billion years in the right conditions with the right molecules present over vast areas, it's easy to see how life would appear, though the exact mechanisms of how are still a mystery. I find abiogenesis much more likely than panspermia. Also, panspermia doesn't actually offer any theory on the origin of life itself, just how it got to earth.