Apple Computer is offering <A HREF="http://news.zdnet.com/2100-1040_22-5384352.html?tag=zdnn.alert">free replacement screens</A> to owners of 15-inch PowerBook models whose machines have a flaw that causes the screen to display white spots. Apple said it will cover machines for two years from the date of purchase, up from the one year of coverage that is provided as part of Apple's standard warranty.