First, the always-on connection is one of broadband's major benefits. Second, it wouldn't help anyway. Most hackers use programs to do their work for them. These programs can check your IP address to see when you're on and when an attack should be mounted. A firewall, which monitors your Internet connection and refuses to let anything suspicious in.
Firewalls come in hardware and software varieties.
Hardware firewalls are boxes that you plug your computer into. Many broadband routers and modems now come with built-in firewall protection. Software firewalls, like BlackICE Defender, are programs that you download from the Internet to your personal computer. It monitors your Internet traffic and compares what's coming in to what's permitted. Most firewalls are set to let Web pages and e-mail pass through, but won' t let telnet or ftp requests in, unless the person has a password.