To prevent viruses from entering a system there are basically just two options. The first of these is to place the computer in a protective 'bubble'. This in practice means isolating the machine; disconnecting it from the Internet or any other network, not using any floppy disks, CD-ROMs or any other removable disks. This way you can be sure that no virus will get into your computer. You can also be sure that no information will enter the computer, unless it is typed in through the keyboard. So you may have a fantastic computer, the perfect data processing machine...but with no data to process. If you're happy with that, your computer will be about as much use as a microwave oven.
The second option is to install an antivirus program. These are designed to give you the peace of mind that no malicious code can enter your PC. But how do they do it? How does the program let you install a game, but prevent a virus from copying itself to disk? Well, this is how it works....
An antivirus program is no more than a system for analyzing information and then, if it finds that something is infected, it disinfects it. The information is analyzed (or scanned) in different ways depending on where it comes from. An antivirus will operate differently when monitoring floppy disk operations than when monitoring e-mail traffic or movements over a LAN. The principal is the same but there are subtle differences.
The information is in the 'Source system' and must reach the 'Destination system'. The source system could be a floppy disk and the destination system could be the hard disk of a computer, or the origin an ISP in which a message is stored and the destination, the Windows communication system in the client machine, Winsock.
The information interpretation system varies depending on whether it is implemented in operating systems, in applications or whether special mechanisms are needed.
The interpretation mechanism must be specific to each operating system or component in which the antivirus is going to be implemented. For example, in Windows 9x, a virtual driver VxD is used, which continually monitors disk activity. In this way, every time the information on a disk or floppy disk is accessed, the antivirus will intercept the read and write calls to the disk, and scan the information to be read or saved. This operation is performed through a driver in kernel mode in Windows NT/2000/XP or an NLM which intercepts disk activity in Novell.
Antivirus products that are not specially designed for operating systems, but are implemented over other applications, have a different interpretation mechanism. For example, in an antivirus for CVP Firewalls, it is the firewall that provides the antivirus with information in order to scan it through the CVP protocol and in the antivirus for SendMail, the MilterAPI filter facilitates information interpretation.
Sometimes an interpretation mechanism is not provided by the antivirus (such as a VxD) or the application (such as the CVP). In this case, special mechanisms between the application and the antivirus must be used. In other words, resources that intercept information and pass it to the antivirus, offering complete integration in order to disinfect viruses.
Once the information has been scanned, using either method, if a threat has been detected, two operations are performed: