I followed the entire development process of Hijack, but I didn't commit every detail to mind. I also haven't had the chance to search the archives yet. I did pull this small note regarding MIME types from usenet:

mime-types are set up so that the extension (m3u, pls, etc.) is
sent as audio/mpegurl (audio/mpeg appears to work for everything but WinXP clients).

I can't recall, in the end, how Mark is sending his m3u's. Also, in case it is just an association problem, have you tried the latest WinAmp releases to see if they will correctly associate under XP? Does IE's options allow you to turn off its media bar (like you can when running IE6 on Win2K)?

[edit:] I just did a quick search and I believe Mark is using audio/x-mpegurl. This is the mime type used by mp3.com. Shoutcast (from Nullsoft) and icecast bth use different types. audio/x-scpls and audio/mpegurl respectively.

Try the streaming from the three places mentioned www.mp3.com and www.shoutcast.com and www.icecast.org and let us know the results.

Bruno