Control Panel -> Internet Options -> Internet Options
Check the box labeled "never dial a connection"
Also, go down to the "Lan Settings" button, and uncheck the "Automatically Detect Settings" which is only used if you have a proxy server.
If you are using a local PPPoE dialer (Enternet 300 or XP PPPoE) on your computah, the setup is quite different.
No. The browsers should all work without modification.
Maybe. If you've changed ISP's between your previous dialup ISP and your broadband ISP, the outgoing SMTP mail servers will probably change.
Hard to tell. You didn't supply the name of the operating system, the make and model numbers of the hardware, your broadband setup, or any background.