Scans of incoming files: Trying to open a file someone sent you? Visiting a Web site that contains a hidden virus? Many antivirus programs will stop these files from entering your system. Often, you'll get a notification that a Trojan is trying to enter your system. When it's detected at this stage, it can easily be blocked.
Scans of the computer: Sometimes viruses and other malware get missed by screening tools, no matter how careful you are. You can perform a system scan using most antivirus software. At the end, you'll be informed of any potential threats that were found and how these threats were resolved.
Tools to clean up computers: Probably the most common complaint among users is that computers seem to slow down after a while. This can be due to a variety of things, and antivirus software often has features to clean up computers and make them run faster.
Tools to prevent spam: Your antivirus software probably has a feature designed to help you cut down on the amount of spam you receive. You just have to "train" the system how to recognize spam messages, a small time investment that is well worth it in the long run. Just imagine if you could stop most of those junk e-mails from ever getting to your inbox in the first place.