I used to know, but now I don't
 

Perhaps it's because I switched to XP and things are a bit different, but how does one change media players and which media formats are played by what player. I'd like to test media on different players, but for each type of media, Windows "knows" which one to use without asking me. Wouldn't it be nice if Windows gave you a list of players you might use if you want to play an AVI or MP3?

Anybody know where this function is?

In explorer

Tools -> options -> Filetypes

In XP, you can just right click the file, click Open With.. and choose a different player. Check the "Always use this program to open this file type box" if you always wanna use that program.