These symptoms have been traced to a device called "ESS Solo-1 PCI AudioDrive", which provides wave audio, midi UART, and FM synthesis services in addition to a software wave table synth.The software synth can be disabled through the Device Manager (in the "System" control panel). The device is listed under "Sound, video and game controllers." The properties dialog for the device has a "Wavetable" page with an "enable" check box.