Vishnu
You can use the following syntax to prevent the "Save as" dialog from appearing, but the form MUST be certified as the exportData function is security restricted (as noted in the Designer help, search on exportData)
xfa.host.exportData("C:/foldername/yourdata.xml", 0);
The form will also need to be Reader Extended to enable Reader to save the data.
Form more information on certified documents, check out:
http://www.adobe.com/security/partners_cds.html
Regards
Steve