Sorry i live in the stone age here (xp).
So if you have vista and above here is how you get the exact serial without modifications.

Code:
 Private Function ff()


        Dim MOS As ManagementObjectSearcher
        Dim MOC As Management.ManagementObjectCollection
        Dim MO As Management.ManagementObject

        MOS = New ManagementObjectSearcher("Select * From Win32_DiskDrive")
        MOC = MOS.Get
        Dim s As String
        For Each MO In MOC
            s = MO("Model")
            s = MO("Name")
            s = MO("SerialNumber")
        Next

    End Function