[JEskandari]

just use FFDshow for divx,xvid ,avi,....... nearly all other codecs like klite ,cccp ,vista codec pack ,........ use old version of it then why not use the latest version and it has the easiest and most uptodate encoder.
for Rm ,Rmvb file if you have no special player in your mind use real media player 11 ,it's the latest from real com and if you use Internet explorer it allow you to download files from sites like youtube.if you want to use another player ,then use the latest version of real alternative.
for QT,Mov .there is only to program that can decode them quick time player ,and quick alternative .i wont recommend quick time player it's the most hideous player i have ever saw ,just use the latest version of Quick alternative.

for players i myself use COWON Media Center - jetAudio Or sometimes real player for the change of the scenery .but windows media player classic and vlc player have their own fans too.
for the sake of the compatibality install CoreAvc but new vrsion of FFDShow must be able to manage those high definition files.
if you want to see the games intros and such without playing ganes i also recommend RADTools to view bink and smacker files and also On2 Vp6 decoder And On2 Vp7 decoder for soome other games.
at last install monkey audio codec but i wont think you will see to much of theses files in last 5 year i only 1 time encountered with them

by installing those you made your own codec pack with the most up todate codecs ,and they are not codec packs to conflict with each other and more important maybe in 5 year 1 time you encounter something that those can't open for you.