Thank, I use those links to get my answer, this was waht I did:
        string path = @"SOFTWARE\Microsoft\NET Framework Setup\NDP";
        List<string> display_framwork_name = new List<string>();
        RegistryKey installed_versions = Registry.LocalMachine.OpenSubKey(path);
        string[] version_names = installed_versions.GetSubKeyNames();
        for (int i = 1; i <= version_names.Length - 1; i++)
        {
            string temp_name = "Microsoft .NET Framework " + version_names[i].ToString() + "  SP" + installed_versions.OpenSubKey(version_names[i]).GetValue("SP");
            display_framwork_name.Add(temp_name);
        }
        return display_framwork_name;
So my output was:
"Microsoft .NET Framework v3.5  SP1"
"Microsoft .NET Framework v3.0  SP2"
and so on....