newbie question

i have an xpcom component that needs to read a string out of a local
XML file .. in a folder somewhere on my machine .. say 'c:\foo
\settings.xml'

I try this:

        xmlDoc = document.implementation.createDocument("", "", null);
        xmlDoc.async = false;
        xmlDoc.load("c:\foo\settings.xml");

but it throws an exception. Moving the xml file alongside the JS file
and calling load("settings.xml") works, but this is not an option.

Q: how do I specify a full path to an XML file for loading??