Playback: Everything sounds like a Piano. MIDI issue?

MuseScore version: 0.9.5
OS: Windows XP

Issue: During score playback, all instruments sound like a piano. The score I'm working on currently includes guitar, violin, cello.

I've previously used Finale Notepad, which reproduced scores with proper General MIDI sounds during playback.

I'm not certain if this is a MuseScore bug or configuration issue, or a configuration issue w/ Windows XP. I've checked the available config screens in MuseScore and Windows, and everything appears to be in order.

Any suggestions on how to resolve this would be appreciated, thanks!

- DrGroove