Umm is Win7 causing this problem: UnauthorizedAccessException?

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C#

Hi All,
I am getting back to doing a project that was put on hold (now it's urgent?). The issues is the program uses a file in the root of the C: drive for logging data.
The code below was used to see if the file is there if not create it. Now I have had to change PC's and Operating Systems as the XP machine this was written on died
Now running on Win7 Pro the code errors at the below points

IniFile ini = new IniFile(INIPath + "\\Widget_emmulation.ini");
           MessageBox.Show("Hello");

           FileName = "C:\\Vapour.txt";

           if (!File.Exists(FileName))
           {
               StreamWriter sw = new StreamWriter(FileName);
        sw.WriteLine("Created By " + txtAdminName.Text + "  " + DateTime.Now + "\n");
               sw.WriteLine();
               sw.Close();

           }
           else
           {
               StreamWriter sw = File.AppendText(FileName);
          sw.WriteLine("Open By " + txtAdminName.Text + " at " + DateTime.Now + "\n");
               sw.WriteLine();
               sw.Close();

           }

Is this due to the code or a security setting. (I can see issues if you have to Admin rights on the machine to save/alter this file!) Why is it firing an UnauthorizedAccessException.....
Glenn
(Hmmm try...catch here we come!)

Further investigation reveals that it is the line
StramWriter sw = new StreamWriter(FileName); so it appears to be the program if it has to creates the file. It does this in both the version run in debug mode and the compiled (.exe) version?