How get I the serial number hard disk windows 7 when permision user is standard

i want protect of my application .so i read serial number hard drive and compare.  the application  has good result in mode administrator user but it has bad result in mode standard user.
i wrote my application with c#.but for read serial number hard i used of  a dll file  that i wrote with delphi. 
<pre>hDevice := CreateFile( '\\.\PhysicalDrive0:', GENERIC_READ or GENERIC_WRITE ,
        FILE_SHARE_READ or FILE_SHARE_WRITE , nil, OPEN_EXISTING, 0, 0 );

i try use of .net. so i using of WMI class(Win32_DiskDrive) but this method has result bad  in mode standard user too.
private string getserial()
        {
            string SerialNumber = "";
            string dataForSerial = string.Empty;
            
                        ManagementObjectSearcher Finder = new ManagementObjectSearcher("Select * from Win32_OperatingSystem");
            string Name = "";
            
            foreach (ManagementObject OS in Finder.Get()) Name = OS["Name"].ToString();

            //Name = "Microsoft Windows XP Professional|C:\WINDOWS|\Device\Harddisk0\Partition1"

            int ind = Name.IndexOf("Harddisk") + 8;
            int HardIndex = Convert.ToInt16(Name.Substring(ind, 1));
            Finder = new ManagementObjectSearcher("SELECT * FROM Win32_DiskDrive WHERE Index=" + HardIndex);
            foreach (ManagementObject HardDisks in Finder.Get())
                foreach (ManagementObject HardDisk in HardDisks.GetRelated("Win32_PhysicalMedia"))
                    SerialNumber = HardDisk["SerialNumber"].ToString();

            // SerialNumber = dataForSerial;
          
            return SerialNumber;
        }
please help me. thank you