参考的网址有：

http://bbs.csdn.net/topics/100033612

http://blog.csdn.net/dengta_snowwhite/article/details/6154199

详细做法：

1、定义结构体

[StructLayout(LayoutKind.Sequential)]
    struct LASTINPUTINFO
    {
        // 设置结构体块容量
        [MarshalAs(UnmanagedType.U4)]
        public int cbSize;
        // 捕获的时间
        [MarshalAs(UnmanagedType.U4)]
        public uint dwTime;
    }

2、定义定时器，结构体对象等

private DispatcherTimer mIdleTimer;
private LASTINPUTINFO mLastInputInfo;

 [DllImport("user32.dll")]
 private static extern bool GetLastInputInfo(ref LASTINPUTINFO plii);

3、初始化

mLastInputInfo = new LASTINPUTINFO();
mLastInputInfo.cbSize = Marshal.SizeOf(mLastInputInfo);

mIdleTimer = new System.Windows.Threading.DispatcherTimer();
mIdleTimer.Tick += new EventHandler(IdleTime);//起个Timer一直获取当前时间
mIdleTimer.Interval = new TimeSpan(0, 0, 0, 1, 0);
mIdleTimer.Start();

4、实现

private void IdleTime(object sender, EventArgs e)
{

　　　　if (!GetLastInputInfo(ref mLastInputInfo))
                MessageBox.Show("GetLastInputInfo Failed!");
            else
            {
                if ((Environment.TickCount - (long)mLastInputInfo.dwTime) / 1000 > 300)
                {
                    MessageBox.Show("no operation for 5 minutes.");
                }
            }

}