If someone has it, could you please help me?
It's been ages since I've seen S-YXG50 (which, indeed, is a great sw-synth), and there was an XP version of it; but there was also a strict evaluation timeout, and the synth didn't work too well in XP anyway.
I suggest trying Timidity (of which I'm pretty sure that there is a Windows version) and maybe Fluidsynth (I'm not sure about it's Windows version), and if you can settle for using SoundFonts in MID file playback, try SynthFont.
Somebody just told me about sfz softsynth that plays sound fonts. I don't know if it will do what you want, but I think it is brilliant - and so easy compared to anything else I have seen. It is freeware, from http://www.rgcaudio.com/sfz.htm
Graham