You could edit the code of the 'Go to checkout' link in your navigation bar and add target="blank" into the <a> tag. You could then edit Act_Order04.html in order to change the 'Done' button to be a...
<A HREF="javascript:window.close();">Finish</A>
...link.
The page underneath (i.e. the page that the checkout was launched from) would not reflect the changes to the online session, however, and also, the 'Cancel' buttons in the checkout will not work properly.
A better bet is to use the 'no frames in checkout' solution provided to you by Darren HERE (http://community.actinic.com/showthread.php?s=&threadid=2972)