Good article on MSDN on kicking off Infopath forms.
There are four methods for invoking an InfoPath form or a browser-based version of the form on a server that is running InfoPath Forms Services. These methods consist of URLs that reference a form template file (.xsn), URLs that reference a form file (.xml) based on a form template, URLs that reference the built-in FormServer.aspx Web page, and those that contain a reference to a custom page that contains the XmlFormView control. The first three kinds of URL support using query parameters to control aspects of the resulting form that includes whether it will be opened in the InfoPath Filler or rendered in the browser, the location where the form will be saved, and the Web page that the user will be directed to when they close the form.