Error Log in asp.net

Question

Hi, I am creating error log - 

public void GetErrorFile(string source, string errormessage)

    {

        string filename = "C:\\Error_" + DateTime.Now.ToString("dd-MM-yyyy") + ".txt";

        if (File.Exists(filename))

        {

            using (StreamWriter writer = new StreamWriter(filename, true))

            {

                writer.WriteLine("Time: " + DateTime.Now);

                writer.WriteLine("Source: " + source);

                writer.WriteLine("Error: " + errormessage);

                writer.WriteLine(" ");

            }

        }

        else

        {

            StreamWriter writer = File.CreateText(filename);

            writer.WriteLine("Time: " + DateTime.Now);

            writer.WriteLine("Source: " + source);

            writer.WriteLine("Error: " + errormessage);

            writer.WriteLine(" ");

            writer.Close();

        }

}

but while creating new error log i am getting error - 

Could not find a part of the path 'my path'.....

I am not getting why this error is coming?