你的启动窗体中没有定义ADO的连接。
如果启动窗体是frmadduser
Private Sub Form_Load()
Combo1.AddItem "system"
Combo1.AddItem "guest"
Dim connectionstring As String
connectionstring = "provider=Microsoft.Jet.oledb.4.0;" & _
"data source=book.mdb"
conn.Open connectionstring
End Sub