Even when a driver is digitally signed, Device Manager displays a “Not digitally signed” message for the driver on a computer that is running Windows Vista or Windows Server 2008
On a computer that is running Windows Vista or Windows Server 2008, you open Device Manager to review the properties of a driver. However, the Digital Signer field on the Driver tab displays the following status message:
Not digitally signed
This behavior occurs even though the driver is digitally signed.
For more information on this issue, including potential causes, workarounds, and resolutions, see: Microsoft KB Article KB955557.