倒数计时器
1）需要：一个Label，2个文本框
2）设计界面：

3）步骤：
Timer控件的Interval 属性改为1000
定义全局计数变量coun，为保证所有过程都能访问该变量，将其申明放在Form1中
Public Class Form1
Dim coun As Long
End Class
输入时间框的TextBox1代码：
Private Sub TextBox1_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles TextBox1.TextChanged
         coun = Val(TextBox1.Text)
     End Sub
显示倒计时时间的TextBox2代码：
Private Sub TextBox1_KeyPress(ByVal sender As System.Object, ByVal e As System.Windows.Forms.KeyPressEventArgs) Handles TextBox1.KeyPress
         If e.KeyChar = Chr(Keys.Enter) Then
             Timer1.Start()
         End If
End Sub
Timer1_Tick响应事||件代码：
Private Sub Timer1_Tick(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Timer1.Tick
         If coun > 0 Then
             coun = coun - 1
             TextBox2.Text = (coun \ 3600) & "小时:" & ((coun Mod 3600) \ 60) & "分:" & (coun Mod 60) & "秒"
         Else
             Timer1.Enabled = False
         End If
     End Sub