IE SP2 on XP SP2 cannot open HTML in a new window from file downlo

Hi,
I am doing a website which does the following:

1. A page within a content frame has a HTML button which when clicked,
submits a form with some data
2. The servlet creates a HTML page and sets the response MIME type to
text/unknown and the content-disposition header to "attachment;
filename=xxx.html"
3. When the user clicks the button, the open/save as prompt is displayed,
which is expected as the 'attachment' content disposition header is set,
however, when the user clicks open, the file opens in the same window as the
website, instead of opening in a new window. Thus, the user is logged out of
the site.
4. IE SP1, opens a new window if the user does the same. Mozilla and
Netscape 7.2 are also working fine and open a new window.

Does anyone know how to get IE SP2 on XP SP2 to open a new window when the
user clicks 'Open' for HTML files. If the user clicks 'Save' and then 'Open'
a new window is opened correctly.

Cheers!
Steve