Try installing the Win XP compatible Driver from: http://www.jamraj.com/2012/09/sony-...
If the issue still persists, Go to Device Manager, look for the unknown USB device, then right click it, click update driver, specify the location (folder) of the webcam drivers you downloaded from Sony website, and then let your computer search for the drivers including that location.
If it's not detected, try to uninstall the unknown USB device, reattach it to your PC, and repeat the procedures above. You can refer to the URL mentioned: http://www.unknowndevicedrivers.com...
Hope this helps
Thanks & Regards