Antivirus software is a must to protect your computer. For PC, there are lots of good "Free of Charge" antivirus software available.
AVAST is one of the highly recommanded antivirus software.
AVG is another good antivirus software.
For those small business having a server running and looking for antivirus protection, WinCAM is one of the widely used open source antivirus software for windows server.
Not matter which antivirus software you are running, update is the most important.
For paid antivirus software, there are Trend Micro; Notron; Symantec.