It's essential that you have antivirus software installed and running on you machine. Not only will it protect your machine and data but it will stop your machine from being used by others to send out spam. To aid in the fight against malware and computer viruses Microsoft have release Microsoft Security Essentials free software that will guards against viruses, spyware, and other malicious software.
There are other good free antivirus products too, so there is no reason not to be running something.
You should only run on antivirus system on your machine.