There are many reasons that a Device Manager will list a device as unknown and hence, there is no foolproof way to troubleshoot the error.
Some of the causes for Unknown Device error are:
* The hardware does not have the requisite software drivers, and Windows is not able to determine the device type
* The driver is not compatible with the OS and hence it is not able to inform Windows about the hardware. This is common if a Windows 9x/ME driver is installed on Windows 2000/XP.
* Each hardware component should normally communicate a basic device ID to Windows. If the hardware does not have this device ID or Windows does not recognise the device ID of the hardware, then the device is listed as unknown.
* Programs can also install virtual devices. Usually, these virtual devices act as a bridge between the software and some hardware function. Sometimes, device driver software for hardware is installed in a non-standard way. When the hardware along with driver software is removed, all the entries in the Registry are notdeleted. First, boot the computer into Safe Mode. If the device is no longer listed in Device Manager, then most likely it was a virtual device driver. Next, check the programs loaded at system startup (Start > Programs > Startup). By trial and error, disable each program till you identify the software that is causing the device to be unknown. Uninstall and reinstall the software. If these steps do not work, or if you suspect that the problem is hardware-related, remove hardware devices from your comput-er one at a time until Device Manager no longer lists the unknown device. Or, if you know what hardware is causing the error, verify that the hardware is working and that you have all the correct and updated drivers for the OS. Install the drivers through the Add Hardware Wizard.