Driver.SwitchTo().DefaultContent();
is for switching frames. Quote from API:
Selects either the first frame on the page or the main document when a page contains iFrames.
You might want to try IAlert
in WebDriver.dll.
For example
try {
// doing your thing
} catch (UnhandledAlertException) {
IAlert alert = driver.SwitchTo().Alert();
Console.WriteLine(alert.Text);
alert.Accept(); // or alert.Dismiss(); depends on your needs
}