When i download a video a type aiv....it wont play.... it says that my media player is missing a codec....what codec should i install??? i need help

Might I suggest (if you don't mind) getting a player with in-built codecs & is generally very reliable: VLC - (click on link) _ (choose nearest mirror site)

To see what it can do, see http://www.videolan.org/vlc/features.html

You can also download extra skins for it; eg: Privately designed skins & also How Can I load A new Skin In VLC Player?

VLC is also good for either audio or video streaming, playing dvd's, vcd's, etc.

have you tried running your computer restore disk for that particular file if it was on the computer originally? You might have just accidentally deleted part of a file or had an error while the media player was being installed. Just remove the program and try redoing the whole thing. I had an old e-machine that would delete the media player program all the time when i rebooted and i would get a similar error message, reloading it always worked

i had this problem too. I got DivX player and i can play all avi files now plus you can download xvid files too which can give you more videos to download.
Just search google for divx player.