You can access the local file system from a regular in-browser Silverlight app, but only through a "Save as" dialog (see
SaveFileDialog Class[
^]), and this dialog must be user-initiated (like on a button click handler).
For example:
private void saveButton_Click(object sender, RoutedEventArgs e)
{
SaveFileDialog sfdlg = new SaveFileDialog();
sfdlg.Filter = "ZIP Files (*.zip)|*.zip";
if (true == sfdlg.ShowDialog())
{
using (Stream fs = sfdlg.OpenFile())
{
}
}
}