My system will dial in even when I have no browser or email software running. The only sure-fire way to prevent the unwanted (or at least unexpected) connections after logging off of my ISP connection, is to initiate another dial-up and then immediately cancel it leaving the "retry-cancel" dialog box on the screen without a response and working "over top" of it.
This is especially annoying when we are using the dial-up line for voice and the system attempts to dial in.