string str = "function () { alert('OMG ITS me'); }";
 
Page.RegisterClientScriptBlock("nameofscript", str);

public static void ShowMessageBox(string message)
        {
            if (!string.IsNullOrEmpty(message))
            {
                if (message.EndsWith("."))
                    message = message.Substring(0, message.Length - 1);
            }
 
            StringBuilder sbScript = new StringBuilder(50);
            //Java Script header
            sbScript.Append("<script type='text/javascript'>" + Environment.NewLine);
            sbScript.Append("// Show messagebox" + Environment.NewLine);
            message = message.Replace("\n", "\\n").Replace("\"", "'");
            sbScript.Append(@"alert( """ + message + @""" );");
            sbScript.Append(@"</script>");           
            // Gets the executing web page
            Page currentPage = HttpContext.Current.CurrentHandler as Page;
            // Checks if the handler is a Page and that the script isn't already on the Page
            if (currentPage != null && !currentPage.ClientScript.IsStartupScriptRegistered("ShowMessageBox"))
            {
                currentPage.ClientScript.RegisterStartupScript(typeof(Alert),"ShowMessageBox", sbScript.ToString());
            }
        }