Hi Matthias,
The only way I know of to do that is to use a redirect.
In IIS you configure default documents, so we already have one for Default.aspx to be a default document, but you can add another one for index.aspx and make it a higher priority default.
Then create a text file in the root named index.aspx and paste in this code, changing the redirect url to point to your sub page.
<%@ Page Language="C#" ClassName="CustomPage.aspx" Inherits="System.Web.UI.Page" %>
<script runat="server">
protected void Page_Load(object sender, EventArgs e)
{
Response.Redirect("ulrtoyoursubpage.aspx");
}
</script>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" >
<head id="Head1" runat="server">
<title>CustomPage</title>
</head>
<body>
<form id="form1" runat="server">
</form>
</body>
</html>
Hope it helps,
Joe