Capturing Function Keys (F5, F3, F1)

 I have a silverlight application that I am developing that's supposed to replace a windows client.  The windows client implements common "browser" functionality internally (F5 refreshing the data in the application).  To this end I need to prevent F5 from causing a browser refresh.
 I found/tweaked javascript so that I can prevent most keys from propagating to the browser if the document has focus (I understand that I cannot, and do not want to capture alt-F4, ctrl-alt-del, winkey, alt-tab, may be others).

 function showDown(evt) {
            evt = (evt) ? evt : ((event) ? event : null);
            if (evt)
            {
                if(evt.keyCode!=17) {
&nbs ...