Get OS Version / Friendly Name in C#

Add a reference and using statements for System.Management, then:

public static string GetOSFriendlyName()
{
    string result = string.Empty;
    ManagementObjectSearcher searcher = new ManagementObjectSearcher("SELECT Caption FROM Win32_OperatingSystem");
    foreach (ManagementObject os in searcher.Get())
    {
        result = os["Caption"].ToString();
        break;
    }
    return result;
}

You should really try to avoid WMI for local use. It is very convenient but you pay dearly for it in terms of performance. Think laziness tax!

Kashish's answer about the registry does not work on all systems. Code below should and also includes the service pack:

    public string HKLM_GetString(string path, string key)
    {
        try
        {
            RegistryKey rk = Registry.LocalMachine.OpenSubKey(path);
            if (rk == null) return "";
            return (string)rk.GetValue(key);
        }
        catch { return ""; }
    }

    public string FriendlyName()
    {
        string ProductName = HKLM_GetString(@"SOFTWARE\Microsoft\Windows NT\CurrentVersion", "ProductName");
        string CSDVersion = HKLM_GetString(@"SOFTWARE\Microsoft\Windows NT\CurrentVersion", "CSDVersion");
        if (ProductName != "")
        {
            return (ProductName.StartsWith("Microsoft") ? "" : "Microsoft ") + ProductName +
                        (CSDVersion != "" ? " " + CSDVersion : "");
        }
        return "";
    }

Add a .NET reference to Microsoft.VisualBasic. Then call:

new Microsoft.VisualBasic.Devices.ComputerInfo().OSFullName

From MSDN:

This property returns detailed information about the operating system name if Windows Management Instrumentation (WMI) is installed on the computer. Otherwise, this property returns the same string as the My.Computer.Info.OSPlatform property, which provides less detailed information than WMI can provide.information than WMI can provide.