Just add ToolTip tool from toolbox to the form and add this code in a mousemove event of any control you want to make the tooltip start on its mousemove
private void textBox3_MouseMove(object sender, MouseEventArgs e)
{
toolTip1.SetToolTip(textBox3,"Tooltip text"); // you can change the first parameter (textbox3) on any control you wanna focus
}
hope it helps
peace