/// <summary>
/// Prüft ob der angemeldete User ein Administrator ist
/// </summary>
public static bool IsAdministrator()
{
 var windowsIdentity = WindowsIdentity.GetCurrent();

 return windowsIdentity != null && 
  new WindowsPrincipal
   (
    windowsIdentity ).IsInRole( WindowsBuiltInRole.Administrator  
   );
}

namespace System.Security.Principal
{
    [ComVisible(true)]
    [Serializable]
    public enum WindowsBuiltInRole
    {
        Administrator = 544,
        User = 545,
        Guest = 546,
        PowerUser = 547,
        AccountOperator = 548,
        SystemOperator = 549,
        PrintOperator = 550,
        BackupOperator = 551,
        Replicator = 552,
    }
}

/// <summary>
/// Prüft ob der angemeldete User die entsprechende Rolle hat
/// </summary>
public static bool IsUserInRole(WindowsBuiltInRole windowsBuiltInRole )
{
 var windowsIdentity = WindowsIdentity.GetCurrent();

 return windowsIdentity != null &&
   new WindowsPrincipal
     (  windowsIdentity).IsInRole(windowsBuiltInRole
     );
}

if (IsUserInRole( WindowsBuiltInRole.BackupOperator ))
  {
   ...
  }