I have some extended SF2 soundbanks in my archive, which I will maybe load in the future.
But being setting up the basic system again, I thought I could rely on the built-in soundbank of the on-board soundchip.
The MIDI plugin doesn't use on-board soundbanks (the vast majority of soundcards don't even have on-board soundbanks). Instead, the plugin uses SF2 soundfonts to provide the sounds, and will default to using the Creative soundfont (ct4mgm/ct2mgm.sf2) if it is present, eg. installed with Creative/SoundBlaster drivers. What soundcard/drivers do you have installed?
Edit : I solved the problem. The basic sf2 file couldn't be found, because in the 'add' dialog the wrong path was used.
It still used the path of my old W2K machine : c:\WINNT\system32\drivers\*.sf2, instead of the XP-path, which is
What soundfont did you find in that directory? The MIDI plugin looks for the Creative soundfont in the system directory (what you've written minus the "drivers" bit).