Hi Brian,
Welcome to the RiffWorks forum!

RiffWorks is compatible with XP and Vista. Make sure you're running all the latest updates.
To get sound through your speakers, RiffWorks needs to be able to identify an ASIO device and it looks like it's picking up your soundcard if you're getting some drum audio. I don't use a Stealth Plug myself but check that RiffWorks can see it by clicking on the 'Audio Setup' button, bottom right in RiffWorks.
When you play your guitar, are the LEDs moving (bottom left)?