How can I change a .txt file's text in c:\windows?

Question

Hello.I want to edit a txt file in c:\windows but I can't...I'm using this code:
StreamWriter sw = new StreamWriter(@"c:\windows\zamma\zamma.txt");
            sw.Write(textBox3.Text);
            sw.Close();
 
What's wrong whit my code? Thanks in advance....
(sorry for my Englisch...)