Some videos won't play
 

I know vaguely about "codecs" but I do not know where to get them. All I know is sometimes the videos won't play and it says in Media Player that codec is not found. Sometimes opening these in DIVX works but not all the time. I have some videos I would love to see! Somebody help! I'm very amatuerish at all this. I know I have Windows XP Professional though. Any help is appreciated!