iframe trouble

I have a sign-in box that is an IFrame, with a form target of "_top", and
the current (parent) page URL as a hidden field.
If sign-in in successful, there is a response.redirect to the page URL,
having set the session variable LoggedInOK=true.

However, if the sign in is not OK, I want the IFrame to be the target, (i.e.
_self, not _top) to display the error.
Is there a way for ASP to control whether it redirects to _top or _self, as
setting the form target in the Login page requires "pre-knowledge" of
whether it will be successful.

Hope this is clear! Thanks in advance, Giles.