Basically, Windows XP uses the DLS (Downloadable Soundbank) library. It can be found in c:\WINDOWS\system32\drivers\gm.dls . It should be possible to convert this DLS soundbank to a SoundFont (*.SF2) using a converter - reportedly, Extreme Sample Converter offers this functionality.
Anyway, I have found this publicly available SoundFont (link). It sounds almost exactly like Windows's standard MIDI soundbank, so it solves my problem :)"