If you use the internet today, you need to protect yourself. There are many things that you can do and none of them are 100% effective. The more you do, the better chance you have of keeping your computer clean. Here's my take on what you can do to help keep your comptuer clean.
Firewalls
Firewalls are the first line of defense from someone trying to get into your PC. This is needed for computers that run any services (web server, etc). Most computers run some type of service (even if you are not aware of it), so a firewall is a good idea. I have 2 firewalls. The first is my Linksys Router (BEFW11S4). This router provide a NAT (Network Address Translation) firewall . A NAT will prevent incoming connections.
For some things, however, your computer will start the transaction to "open the door." For these type of things, I use Zone Alarm from Zone Labs. This will prevent applications on your computer from accessing the internet as well as further restricting what can access your computer.
Anti-Virus
There are many products that you can run as an Anti-Virus scanner. I run Norton Anti-Virus with Live Update updating every week. As with anything, this is not 100%, but it does a very good job. I have also ran HouseCall from TrendMicro. Between these two, I usually keep the virus activity to a minimum. Scanning any attachments that you receive via e-mail before you open it will help dramatically - if you do not know the person that sent you the attachment, I would delete it without worry.
Prevention
As with anything, an ounce of prevention is worth a pound of cure. Preventing viruses from getting into your computer is even a better solution. Most viruses exploit "holes" in the software that we use. And two common targets for these "hole" attacks are Microsoft's Internet Explorer and Microsoft Outlook. I prefer to use replacements for these two pieces of software. I use Lotus Notes for my e-mail and Firefox for my web browser. The same people that make Firefox also have Thunderbird for e-mail.
Ad-Ware/Spy-Ware
To further enhance my scanning, I run Lavasoft's Ad-Aware. This software helps to protect from known data-mining, aggressive advertising, browser hijackers, etc.
I hope this helps you decide what you need to keep your computer clean. Even running all of these things will not guarantee that you will not have problems. Good luck!