Short Answer: Yes.
Long Answer: I would recommend either a Firewall or using a cable router (in addition to your Virus Checker). Both will stop attempts to connect to your machine from outside & will stop the majority of hacking attempts. What they don't prevent is trojans which you receive in your email, or via web-download. Your virus checker will get those
The Firewall is the best bet if you are using a single computer, directly attached to the cable modem. However it can be tricky to configure so that it doesn't give false alarms.
The Cable-Router is the best solution if you want to attach more than one PC to your Cable-Modem. It provides the required protection 'out-of-the-box'. You can use a Firewall program as well, but personally I think that is a little too paranoid.
Alan