Matt is correct. There is no way to physically force this on the user or to change the settings since they reside on IE. If you need this functionality there is an activeX control out there that will do this for you.
That control can be found here: http://www.meadroid.com/scriptx/intro62.asp
Read this if you want to know how to get a correct reply for your question:
^^Took that from planoie's profile^^
^^Modified text taken from gbianchi profile^^
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