Visual Basic - GetVolumeInformation no funciona

Estoy usando la API "GetVolumeInformation", la cual me regresa como resultado el número de serie del HD, el problema es que cuando la llamo me regresa el número cero y ademas de ello el parámetro de referencia no regresa nada tampoco. La función que he escrito es la siguiente:

Function ObtenerVolHD()
'Numero serie de HD
Dim A As String
Dim VolumeNameBuffer As String * 256
Dim tamaVolume As Long
Dim SerieNum As Long
Dim Maximo As Long
Dim Band As Long
Dim SystemaBuf As Long
Dim TamNameSize As Long
Dim v As Long
Dim Etiqueta As String

A = "C:\"
v = GetVolumeInformation(A, VolumeNameBuffer, tamaVolume,
SerieNum, Maximo, Band, SystemaBuf, TamNameSize)

ObtenerVolHD = (Left(Hex(SerieNum), 4) & "-" & _
Right(Hex(SerieNum), 4))
End Function

Cual es el problema?

Les agradezco de antemano

Pruebalo con esto:
Private Declare Function GetVolumeInformation Lib "Kernel32" Alias "GetVolumeInformationA" (ByVal lpRootPathName As String, ByVal lpVolumeNameBuffer As String, ByVal nVolumeNameSize As Long, lpVolumeSerialNumber As Long, lpMaximumComponentLength As Long, lpFileSystemFlags As Long, ByVal lpFileSystemNameBuffer As String, ByVal nFileSystemNameSize As Long) As Long
Private Const MAX_FILENAME_LEN = 256
Public Function DriveSerial(ByVal sDrv As String) As Long
Dim RetVal As Long
Dim str As String * MAX_FILENAME_LEN
Dim str2 As String * MAX_FILENAME_LEN
Dim a As Long
Dim b As Long
Call GetVolumeInformation(sDrv & ":\", str, MAX_FILENAME_LEN, RetVal, a, b, str2, MAX_FILENAME_LEN)
DriveSerial = RetVal
End Function
Private Sub Form_Load()
MsgBox "Serial of drive C is " & DriveSerial("C")
End Sub
si quieres mas informacion:
http://www.mentalis.org/apilist/GetVolumeInformation.shtml

En la web del guille tenés un ejemplo

http://elguille.info/vb/VB_API01.HTM#api004

Saludos Ruri