Access is denied error in form.submit() in window.onbeforeunload

In my modal dialog windows I open a page with a frameset.
The frameset contains two .htm pages, and the bottom page contains
a form that needs to be submitted onbeforeunload.
On Vista with IE8 you do not need to specify a target for the form,
on Windows7 with IE8 you need to specify a target to avoid the access is
denied error.

Problem is that the only target possible is "_blank", because the frameset
is closing, which results in a blank page being opened (which is normal) and
this is not want one would want, because the submit of the form needs to
happen
in background.

Is the way forms are submitted in IE8 different between the older OS
versions and Windows7 ?
Why is a submit to "_self" possible in a closing modal dialog on Vista with
IE8 and not on Windows7 with IE8 ?

Best regards,
--
Renzo Callant