if they are WMA (ie unprotected/unlicenced) then i think you can just drop them into itunes, and they will convert. (although the more i think about this, the more i'm thinking it's only the WinXP version of iTunes that can do this. if not, try ffmpegx (google it), which is a good cross-coder for many video/audio formats.
if it's WMP (protected, licenced, DRM) then there's no real legal way to do it (if any way).
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