Problem with DIrectory permission when reseting it

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C#

DirectoryInfo myDirectoryInfo = new DirectoryInfo(textBox1.Text);
DirectorySecurity myDirectorySecurity = myDirectoryInfo.GetAccessControl();
string User = System.Environment.UserDomainName + @"\" +
                                           comboBox1.SelectedItem.ToString();
myDirectorySecurity.ResetAccessRule(new FileSystemAccessRule(User,
                              FileSystemRights.Read   , AccessControlType.Deny ));
myDirectoryInfo.SetAccessControl(myDirectorySecurity);
MessageBox.Show("Permissions Altered Successfully");

I have written this program to lock a folder.. and it is doing so.. but when i try to unlock any of the folder locked by this except the folder from c: drive .. it gives a message. unauthoruzed access exception... why i m getting this message.. how to fix this??? thanks in advance for ur solution

[edit]Code block added, "Treat my content as plain text..." option disabled. "\\" changed to @"\" to prevent confusing syntax colorizer. - OriginalGriff[/edit]