I recently had the same problem, my solution was I bought One Man Band software which included it's own synth.
The software was inexpensive and is also a host if you want to get into that. It also allows you to play Yamaha styles on your computer.
I bought OMB because it was so simple to setup the price was well worth the time it saved me in trying to figure out drivers and soft synths and all that stuff I didn't want to learn.
Doesn't the keyboard come with some sort of software? Mine came with cakewalk le but I ditched in favor of the above simple method.

P.S. the soft synths and hosts and vsti's required way more time than I wanted to invest, I wanted to plug in and play. The lazy way out.

[This message has been edited by mikeathome1 (edited 04-10-2008).]