Combo box selected index

hey, i have a combo box on my form and have defined a method for the SelectedIndex changed event.I load the combo box with values and set the index to minus one.in the watch window the value changes to minus one and the SelectedIndex changed event fires and i can step through it.Nothing changes the index again but when the form appears on screen the first index is selected i.e. index 0. this happens every time. I also have a clear button which sets this combo box to have an index of minus one but when i press the combo box the index re-sets to 0.i have to press the clear button a second time to re-set the index to minus one. Why does this happen does anyone know? thanks.