Texbox OnKeyPress event thru javascript executes a button click event

Hi.
 
I have an application consists of 2 textboxes, 2 buttons, a modalpopup extender.
 
Button1 and Textbox1 is in a modalpopupextender that is shown when a button is click (not shown here)
 
I added an OnKeyPress event on my textbox (Textbox1) on Page Load that will execute a javascript on client side code which calls a button (Button1) click event.]

protected void Page_Load(object sender, EventArgs e)
    {       
               if (!Page.IsPostBack)
        {
            this.TextBox1.Attributes.Add("onKeyPress", "doClick('" + Button1.ClientID + "',event)");
        }
    }

This is the Javascript codes, this will call and execute the click event of Button1

 function doClick(buttonName, e) {
            //the purpose of this function is to allow the enter key to 
            //point to the correct button to click.
            var key;
            if (window.event)
                key = window.event.keyCode;     //IE
            else
                key = e.which;  //firefox
            if (key == 13) {
                var btn = document.getElementById(buttonName);
                if (btn != null) { //If we find the button click it
                    btn.click();
                    event.keyCode = 0
                }
            }
        };

This is the Button1 Click event

protected void Button1_Click(object sender, EventArgs e)
    {
        if (Textbox1.Text.ToString() == "ITDevt")
        {
            Button2.Visible = true;
            Textbox2.Visible = false;
Button2.Visible=true;
Button1.Visible=false;
            popup.Show();
        }
        else
        {
            popup.Hide();
            Textbox1.Text = "";
        }
    }

Assume that the modalpopupextender is shown, so I entered a text in Textbox1 and presses Enter key on keyboard (so the OnKeyPress event is rendered).
When I trace the Button1_Click event, the code block runs. But the popup (modalpopupextender) hides.
 
Can you help me show again the modalpopupextender and hide Button1 and Textbox1 then show Button2 and Textbox2?