You are selecting row on click this when checkbox is clicked then too also Row click event is fired.
There is no way to avoid it, except that change onclick to odblclick
e.Row.Cells[i].Attributes["ondblclick"] = Page.ClientScript.GetPostBackClientHyperhttp://www.aspforums.net/this.gvTask, "Select$" + e.Row.RowIndex;
So that when row is double clicked it will be selected