what you mean by "the default program for web authoring".
There will be no default program - each application has its own file format, but as they both create HTML files you can open a web in either of them. A site that is authored in Dreamweaver can be opened in FrontPage and vice versa. FrontPage has certain dynamic components that require extensions to be installed on a web server before they will work - animated hover buttons for instance - and these will not function in Dreamweaver. Otherwise there'll be no real problems.
If you want to work on a FrontPage site in Dreamweaver you must import it - you can't simply click on an HTML file and expect it to open an authoring program. If you click an HTML file it will, by default, open in Internet Explorer.