If you encounter this error when scanning a Vista (or Windows 2008) machine you will need to enable the Policy:

Allow Remote Access to the PnP Interface

Run gpedit.msc (your Group Policy Editor) and navigate to

Computer Configuration\Administrative Templates\System\Device Installation. Change the appropriate policy (Allow Remote Access to the PnP Interface to Enabled.

Note, you will not be able to set this policy via a Windows 2003 Group Policy Editor (since this policy was introduced In Windows Vista). You can set it via Windows 2008 or Windows Vista Group Policy Editor. A reboot of the target is needed after this change has been made.