error web service access is denied cuando se envia un documento

System.Security.Cryptography.CryptographicException: Access is denied.

   at System.Security.Cryptography.CryptographicException.ThrowCryptogaphicException(Int32 hr)
   at System.Security.Cryptography.Utils._CreateCSP(CspParameters param, Boolean randomKeyContainer, SafeProvHandle& hProv)
   at System.Security.Cryptography.Utils.CreateProvHandle(CspParameters parameters, Boolean randomKeyContainer)
   at System.Security.Cryptography.RSACryptoServiceProvider.ImportParameters(RSAParameters parameters)
   at WebService.ted_signature.getRSA(String t)
   at WebService.ted_signature.generar_firmar_ted(String contenido_a_firmar, String rsask)
   at WebService.ted_xml.crear_ted(String nodo_caf, String rsask, String rut_emisor, String dte, String folio, String fecha_emision, String rut_receptor, String razon_social_receptor, String monto_total, String item1_detalle, String ts)
   at WebService.Service1.firmar_xml(Byte[] f, Char ambiente)

Solución: dar permisos al usuario Everyone/Todos en la carpeta C:\Documents and Settings\All Users\Application Data\Microsoft\Crypto\RSA\MachineKeys