WebService gemme fil problem

Jeg har en webservice der skal kunne tage imod en fil og gemme den på disken. En slags fileupload. Min metode til at uploade filen ser således ud:

[WebMethod]
        public void PutFile(byte[] buffer, string filename)
        {
            BinaryWriter binWriter = new BinaryWriter(File.Open(Server.MapPath(filename), FileMode.CreateNew, FileAccess.ReadWrite));
            binWriter.Write(buffer);
            binWriter.Close();
        }

Når jeg så forsøger at gemme filen på disken får jeg følgende fejlbesked som jeg ikke helt ved hvordan jeg skal komme ud over:

System.Web.Services.Protocols.SoapException: Server was unable to process request. ---> System.UnauthorizedAccessException: Access to the path 'C:\Documents and Settings\brian_h\My Documents\Visual Studio 2005\Projects\FTPCVS\WalrusWebService\Test.aspx' is denied.
  at System.IO.__Error.WinIOError(Int32 errorCode, String maybeFullPath)
  at System.IO.FileStream.Init(String path, FileMode mode, FileAccess access, Int32 rights, Boolean useRights, FileShare share, Int32 bufferSize, FileOptions options, SECURITY_ATTRIBUTES secAttrs, String msgPath, Boolean bFromProxy)
  at System.IO.FileStream..ctor(String path, FileMode mode, FileAccess access, FileShare share)
  at System.IO.File.Open(String path, FileMode mode, FileAccess access)
  at WalrusWebService.Walrus.PutFile(Byte[] buffer, String filename) in C:\Documents and Settings\brian_h\My Documents\Visual Studio 2005\Projects\FTPCVS\WalrusWebService\Walrus.asmx.cs:line 35
  --- End of inner exception stack trace ---