implement the MouseHover/MouseLeave event of the control in question. Once done, then just do your thing!
so if we had a button on the form and we had a label which shows us the friendly "tooltip", we implement the MouseHover to show us the text. Click on the button, view its events in the properties and select the lightning symbol - this will give you a list of events supported by that control in question. Then double click the MouseHover event. In here we do our code! so something like...
private sub button1_MouseHover(object sender, EventArgs e)
this.theLabel.Text = "I'm hovering over button1";
go back to the designer view and double click the MouseLeave event to create the other event. Something like...
private sub button1_MouseLeave(object sender, EventArgs e)
this.theLabel.Text = String.Empty;
is this what you are after