An antivirus program is a system for analyzing information
An antivirus program is a system for analyzing information, if it finds that something is suspicious file in a system then diagnose it and correct 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.
More at http://www.net-security.org/article.php?id=485