Dont get me wrong, peanut butter has fat in it, but it's not really that bad for you, peanuts thamselves have alot of fat in them, but it's not the bad kind of fat that's in alot of other foods you eat. If your trying to gain weight the most healthy way, peanut butter is not a bad food to put on your list.
From http://www.menshealth.com/cda/article.do?site=MensHealth&channel=nutrition&category=abs.diet&conitem=b72a99edbbbd201099edbbbd2010cfe793cd____&page=3