HighMem

If you added an entry to the "Open with" menu of nautilus (i.e. Gnome file-manager), you currently can not customize the string that is displayed. Maybe you added two entries "xmms" and "xmms -e" (enqueue) for mp3-files. Then both will show up as "xmms" in the "Open with" menu. Thats odd, but customization is possible even though the Gnome makers keep hiding interesting options like that. (Maybe Linus is right about KDE/Gnome? - but then... Gnome is still much more beautiful...)
Having said that, here's the obscure location where nautilus stores your custom "Open with" entries:

~/.local/share/applications

1. Install gxine
2. Install w32codecs (google for it)
   
3. Create symbolic links to the following files in your Mozilla plugins directory (could be ~/.mozilla/plugins): gxineplugin.a gxineplugin.la gxineplugin.so (might be under /usr/lib/gxine)
   This crashed frequently. Install mozplugger instead!
   
4. Delete the pluginreg.dat
5. Add the following to /etc/mozpluggerrc or modify anything similar to
   

   application/x-mplayer2: wmv,asf,mov: Windows Media
   video/x-ms-asf: asf,asx,wma,wax,wmv,wvx: Windows Media
   video/x-ms-wmv: wmv: Windows Media
   stream noisy ignore_errors: gxine "$file" 

6. Start Firefox, Quit Firefox -> pluginreg.dat was re-created
7. Done

Note: The lines above may already be present but pointing to mplayer. So if you have mplayer installed you need to modify only the last line (i.e. the command to be executed).