Maybe you should re-install.
I'm not a winamp user, but I just downloaded WinAmp Portable from Softpedia :
http://www.softpedia.com/get/PORTABLE-S ... namp.shtml
And extracted with Universal Extractor, it doesn't have any of the folders & files that are instructed to be deleted in the database here, so I guess they have cleaned up the portable version some.
I did the right-click Open With and it worked for me.
I don't understand why you would get a message that xxx.MP3
is not a valid application, if you selected the winamp EXE
file to open your mp3 file. If anything, it should have said "winamp.exe" is not a valid application.
Or did I mis-understand what you wrote?