What I do in such circumstances is run a little piece of freeware called "Unknown Device Identifier" which does a pretty good job of telling me what device is not working properly, identifying the manufacturer, which allows me to look up the support site and get the driver necessary to correct the problem. I've found that to be the quickest and surest way to resolve such issues.
You can also try double clicking the device in device manager and then selecting "update driver" and see if Windows can locate it on your hard drive or search the internet [providing you have a working connection] to find the appropriate driver and install it. I find it works about half of the time.
There is are other trouble shooting methods and the Microsoft Knowledge base does the most straight forward job of defining them so rather than reinvent the wheel I'll present them as written there:
The main reason for Unknown Device to
display in your Device Manager is because you have a device that is not
working correctly. To fix this issue, obtain an updated driver for the
device.
Use the following methods to enable Windows to recognize your device.
Method 1: Windows Update
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Visit the following Microsoft Web site to see whether the Web site
autodetects an updated driver for your device. Install all recommended
drivers.
http://www.windowsupdate.com
(http://www.windowsupdate.com)
Method 2: Download and install the latest driver from the Internet
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When
you download a driver from the Internet, download the Windows XP
version of the driver when multiple Windows versions are listed.
Use one of the following options:
- Visit the Web site of the computer manufacturer if the device came with the computer.
- Visit the Web site of the device manufacturer if the device was installed after you purchased your computer.
- Visit the Web site of the device manufacturer if the device
was preinstalled on your computer and the computer manufacturer does
not have an updated driver.
Look for a Drivers section or a Download
section on the manufacturer’s Web site, and search for the device name.
Download and install the Windows XP version of the driver.
Advanced user section
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This section is intended for advanced computer users. If you are not
comfortable with advanced troubleshooting, you might want to ask
someone for help or contact support. For information about how to do
this, visit the following Microsoft Web site:
http://support.microsoft.com/contactus
(http://support.microsoft.com/contactus)
Causes of unknown devices in Device Manager
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- The device does not have a device driver:
When a device driver for a device is not available, Device Manager displays the device as Unknown Device and puts it in the Other devices
branch. This is very common with universal serial bus (USB) and
Institute of Electrical and Electronics Engineering (IEEE) 1394
composite devices. Also, a status of Error Code 1 or Error Code 10 may be displayed when you view the properties of the device in Device Manager.
Note
Most USB and IEEE 1394 devices function correctly without additional
device drivers because they are configured by the drivers that are
included with Windows for these bus types. However, an additional
device driver is needed if the device does not fit the defined and
supplied Windows class drivers. If the bus cannot identify the device,
it interprets the device as a composite device and then reports it as
such in Device Manager.
- You are using a Microsoft Windows 98 or Microsoft Windows 95 Device Driver:
You
cannot use virtual device driver (.vxd) files that are common to
Windows 98 or to Windows 95 drivers in Windows XP. If you try to
install them on your Windows XP-based computer, the device may be
listed as unknown in Device Manager. This typically occurs when the
device driver manufacturer does not correctly differentiate between the
two drivers or assumes that Windows XP can use Windows 98 or Windows 95
.vxd files.
- Unrecognized Device ID:
Every hardware device has a special identifier that is used by Plug and
Play. This identifier can include several different types, such as
vendor ID,
device ID, subsystem ID, subsystem vendor ID, or revision ID. If a
device ID is not present, or your Windows XP-based computer does not
recognize the device ID, Device Manager may list the device as unknown.
Note
Software programs that require virtual hooks into hardware may create
these devices. Additionally, devices that bridge between bus types,
such as a device driver that enables a parallel port device to emulate
a SCSI or an AT Attachment Packet Interface (ATAPI) bus, are also known
to generate an unknown-device response in Device Manager.
- Faulty Hardware or Firmware:
Faulty hardware or firmware may cause a device to be displayed as Unknown Device
in Device Manager. Software-only device drivers do not expose a device
ID, and there is no standard method for installing these devices. Some
manufacturers install the device by using the InstallShield
installation program or a similar method. Be aware that software
installed by other methods may not be completely removed when the
device is removed in Device Manager. Therefore, you may have to check
the registry to verify that all entries are removed.
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