Video formats/players question

I just bought windows vista ultimate 64, i loaded all my programs back on it, Windows media player, Real Player, VLC media player, quick time, Divix player.... Anyway I have a lot of videos, some in AVI format and some in WMV format... For the WMV format in the folder that they are in i get a sample cap for the icon of the video so i know what it is.. But for what it seems most of the AVI files it just shows the default real player icon and no picture of the video... I had all the same programs on windows XP and it worked just fine and now it dosent... Of course i could get a converter but im not aware of any that are free and i would rather just try and find a solution for the problem other then converting all of the videos. Ive also tried some codecs too but no luck... any suggestions?