form submit and redirect?

From: Adam (adam_at_firstmanmedia.com)
Date: 07/29/04 

Date: Thu, 29 Jul 2004 19:38:51 GMT

I have a php page with among other things, some javascript to do a quick
validation of the form field inputs, a small flash movie and a form which
submits 4 fields to email. Everything works fine.

All I want to do now is make a new page load in that window when the form
is submitted. I though all that I needed was:

header ("location: coupondone.php");

But when submitting the form I get the error:

Warning: Cannot modify header information - headers already sent by (output
started at /home/fusion/public_html/coupon.php:5) in
/home/fusion/public_html/coupon.php on line 108

where coupon is the page with the form on it and coupondone.php is the one
I am trying to load. Everything else still works fine but the new page
won't load.

How do I do this?