500 files? If they're all in the same directory, just navigate there with the command line and type:
attrib -s -h *.mp3
If they're al in different directories, but contained lower down in a common folder, say c:\mp3\old and c:\mp3\new, go to the higher directory (c:\mp3) and type:
attrib -s -h *.mp3 /s
The /s makes it go through all of the subdirectories, in addition to the current one.
Of course, this is based on win2k, since I don't have XP. :\