With this Microsoft Fixit

Microsoft Fixit

“Automatically diagnose and fix issues that Windows Phone 7, Zune or Zune HD device is not recognized or detected when connected to a pc.”
What it fixes...
  • Driver not installed
  • Out of date drivers
  • Device not found errors
  • Device and Class Filter blocking device detection

 

http://support.microsoft.com/mats/Windows_phone_7_and_Zune_diagnostic/en-us

Rob