To dynamically add script tags to the head section of an HTML page based on conditions at page load use the following code.

This one adds a link to an external .js file:

if(UseUrchin)
      {
        HtmlGenericControl Include = new HtmlGenericControl("script");
        Include.Attributes.Add("type", "text/javascript");
        Include.Attributes.Add("src", "/__utm.js");
        Page.Header.Controls.Add(Include);
      }

This one adds actual JavaScript code inside the script tag:

if(UseUrchin)
      {
        HtmlGenericControl Include = new HtmlGenericControl("script");
        Include.Attributes.Add("type", "text/javascript");        Include2.InnerHtml = "alert('We added it to the head');";       
        Page.Header.Controls.Add(Include);
      }