Can't create a text file in C#

Question

My app has a Login Form. 

Following code which I created a text file is not correct. I don't know why. 

Error is Access denied in C:\Windows\Apple.txt

And i want this text file to log usernames when user changed his username

Please help me with correct code.

thx C-SharpCorner guys......

using System.IO;

string fileLocation = @"C:\Windows\Apple.txt";

                

if (!File.Exists(fileLocation))

{

       FileStream xFile = new FileStream(fileLocation, FileMode.CreateNew);

       StreamWriter sw = new StreamWriter(xFile);

       sw.Write(this.txtUsername.Text);

       sw.Close();

}