Are you using a multiselect or single select listbox ?
If it's not multi select, you can use....
CODE
Dim intX As Integer
Me.txtLbox1 = Me.lbox1.Column(0)
intX = Me.txtLbox1.ListIndex
Me.lbox1.Selected(intX) = False
DoCmd.OpenForm "frmLessonTaskDisplay", , , , acFormReadOnly, acDialog