在模块中加入下列声明：

Public 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
'得到某一磁盘分区的信息，如C：

窗体代码如下：
Option Explicit
Private Regid, Localid As Long

Private Sub CmdLocalID_Click()
  '根据C盘序列号得到原ID
  Dim Driver, VolName, Fsys As String
  Dim volNumber, MCM, FSF As Long
  Driver = "c:"
  Dim res As Long
  res = GetVolumeInformation(Driver, VolName, 127, volNumber, MCM, FSF, Fsys, 127)
  'volNumber是C盘序列号
  Localid = volNumber / 2 + 123456789
  Text1.Text = Localid
End Sub

Private Sub CmdRegID_Click()
  '根据原ID算出注册ID
  If IsNumeric(Text1.Text) Then
    Regid = CLng(Text1.Text) / 4 * 3 + 987654321
  Else
    'error
  End If
  Text2.Text = Regid
End Sub

Private Sub CmndCheckID_Click()
  '验证注册ID
  Dim Driver, VolName, Fsys As String
  Dim volNumber, MCM, FSF As Long
  Driver = "c:"
  Dim res As Long
  res = GetVolumeInformation(Driver, VolName, 127, volNumber, MCM, FSF, Fsys, 127)
  Dim Tid As Long
  Tid = volNumber / 2 + 123456789
  If Regid = Tid / 4 * 3 + 987654321 Then
    MsgBox "正确！"
  Else
    MsgBox "错误！"
  End If
End Sub

为便于演示，我在窗体上用了两个文本框三个按钮，请根据情况灵活使用，可以在用户端算出原ID，发给你，你算出注册ID再发给用户，验证当然是在用户端啦。ID算法很简单，只作示范。此注册ID只能在这一台机器上使用，对于非高手用户来说是足够啦。

注：如用户格式化C：盘后需重新获得ID。