How to get browser to write a file to local disk

I've been asked to help solve a browser issue that is thorny at best if 
not impossible due to security.

There is a browser based application written in Flash Action Script that 
needs to write an XML file to the local disk. Picture a salesman with a 
USB flash drive that he can use at a customer's site. All the files are 
on the flash drive and a remote server is never contacted so the 
application is completely client side. To start the application, a 
browser is fired up and the local file opened from the flash drive which 
is a form with several list boxes that the customer can choose various 
options and then submit the form. What should happen is a XML file is 
then written to disk which the application uses in several ways further 
downstream including applying an XSLT transformation to display the 
results.

The specification targets IE6, IE7 and Firefox running on XP and Vista. 
The catch is that none of these browsers allows files to be written to 
disk for security reasons regardless if Java applets, JavaScript, 
ECMAScript, etc. are used. So the problem is once the form is submitted 
and the Flash Action Script has the output XML ready, how to circumvent 
security and get the XML file written to preferably the same drive and 
directory the application was launched from.

Having never programmed in Flash Action Script, I was hoping that Action 
Script could call an executable and pass either the XML or form 
parameters to create the XML. The initial feedback to this was "big 
doubts" to paraphrase nicely.

Is what I described possible? Are there other solutions for 
accomplishing this? The application is nearing completion and this piece 
is becoming trickier than expected.

Thanks in advance!

-Jim