keyboard audio-key reassign

I'm running a DELL Optiplex GX620 and also use the Audio-Keys at the keyboard very heavy. Until today, it always started with "winamp", which was fine ... but nowadays the "Windows Media Player" pops up. How can I change this back? I already checked the standard-application for sound-files and this is winamp. how can i get back to that, while pressing this key? thanks in advance. i'm using win vista and Dell Enhanced Multimedia Keyboard ...