there are a number of software programs that will allow you to capture stills from movie files some of these are free and some will cost anywhere from a few $ to several 100$'s.
Do a search on google and look for something that will suit your OS. Many of those that cost do have a free trail that you try first before you buy.
My New Avatar is in memory of Nancy my darling wife of 10 years who passed away on Monday November the 16th 2009 after an illness
My photo gallery http://thekiwi.org/photography/index.php