Microsoft Knowledge Base Article 314464 contains the following INTRODUCTION:
Device Manager displays a list of all devices that are installed on your Microsoft Windows XP-based computer. When you view device information in Device Manager, you may see a device listed as Unknown Device next to a yellow question mark. It may be hard to determine the cause of this unknown device, because there are few indications of what generates it. This article explains the possible reasons that an unknown device may appear in Device Manager.