Hoping to inquire upon the computer savvy in here

I've got some divx video's (680-700 megs in size) and they load up and the
sound is fine, but no picture. This happened before and i downloaded newer
codecs and voila.. it works.

Now im having the same problem but I thought i had the newest codec. I
downloaded some more codecs but no good.

Does anybody know how to determine what codec i need or where is a good
source of them ? ( or is there something else wrong maybe ?)

I got a radeon 32 meg ATI All in wonder if thats relevant
Thx for any input