Private Declare Function GetVolumeInformation& Lib "kernel32" _
    Alias "GetVolumeInformationA" (ByVal lpRootPathName _
    As String, ByVal pVolumeNameBuffer As String, ByVal _
    nVolumeNameSize As Long, lpVolumeSerialNumber As Long, _
    lpMaximumComponentLength As Long, lpFileSystemFlags As _
    Long, ByVal lpFileSystemNameBuffer As String, ByVal _
    nFileSystemNameSize As Long)
    Const MAX_FILENAME_LEN = 256


Private Sub Command1_Click()
    Label1.Caption = SerNum("C") 'C is the standard harddisk
End Sub


Public Function SerNum(Drive$) As Long
    Dim No&, s As String * MAX_FILENAME_LEN
    Call GetVolumeInformation(Drive + ":\", s, MAX_FILENAME_LEN, _
    No, 0&, 0&, s, MAX_FILENAME_LEN)
    SerNum = No
End Function