MusikMan, don't install the drivers if you don't have the soundcard on board. I can imagine that the S-YXG50 version that you have is restricted to work only with the soundcard that it came with. You can try to download a trial version at http://www.yamaha.co.uk/xg/html/midplug/m_mid8.htm to see if that works any better.

By the way it seems that there can be a latency problem when you use a software synth in a live performance tool. But that can depend on how fast your PC is.

JM