Need help with this code, trying to open a new window with a button.

I have a code built to handle a bunch of checkboxes, and then a submit button. When a certain box is checked the button pressed, it should open a new window for the page. It works, but I have to click the button twice to get it to work. The first time it just reloads the checkbox page, and the second time it finally opens the new window. What am I missing?
My button is <asp:Button ID="Submit" Text="Submit" OnClick="Get_CheckBoxes" runat="Server" />
code behind:
public void Get_CheckBoxes(object Src, EventArgs Args)
{
if (C ...