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Anti-virus software typically uses two different techniques to accomplish this:

• Examining (scanning) files to look for known viruses matching definitions in a virus dictionary
• Identifying suspicious behavior from any computer program which might indicate infection

Most commercial anti-virus software uses both of these approaches, with an emphasis on the virus dictionary approach.

1 Virus dictionary approach

In the virus dictionary approach, when the anti-virus software examines a file, it refers to a dictionary of known viruses that the authors of the anti-virus software have identified. If a piece of code in the file matches any virus identified in the dictionary, then the anti-virus software can take one of the following actions:

1. delete the infected file
2. quarantine the file (such that the file remains inaccessible to other programs and its virus can no longer spread)
3. attempt to repair the file by removing the virus itself from the file

To achieve consistent success in the medium and long term, the virus dictionary approach requires periodic (generally online) downloads of updated virus dictionary entries. As civically minded and technically inclined users identify new viruses "in the wild", they can send their infected files to the authors of anti-virus software, who then include information about the new viruses in their dictionaries.

Dictionary-based anti-virus software typically examines files when the computer's operating system creates, opens, closes or e-mails them. In this way it can detect a known virus immediately upon receipt. Note too that a System Administrator can typically scheduled the anti-virus software to examine (scan) all files on the user's hard disk on a regular basis.

Although the dictionary approach can effectively contain virus outbreaks in the right circumstances, virus authors have tried to stay a step ahead of such software by writing " polymorphic viruses", which encrypt parts of themselves or otherwise modify themselves as a method of disguise, so as to not match the virus's signature in the dictionary.

2 Suspicious behavior approach

The suspicious behavior approach, by contrast, doesn't attempt to identify known viruses, but instead monitors the behavior of all programs. If one program tries to write data to an executable program, for example, the anti-virus software can flag this suspicious behavior, alert a user and ask what to do.

Unlike the dictionary approach, the suspicious behavior approach therefore provides protection against brand-new viruses that do not yet exist in any virus dictionaries. However, it also sounds a large number of false positives, and users probably become desensitized to all the warnings. If the user clicks "Accept" on every such warning, then the anti-virus software obviously gives no benefit to that user. This problem has worsened since 1997, since many more nonmalicious program designs came to modify other .exe files without regard to this false positive issue. Thus, most modern anti-virus software uses this technique less and less.

2.1 Other ways to detect viruses

Some antivirus-software will try to emulate the beginning of the code of each new executable that the system invokes before transferring control to that executable. If the program seems to use self-modifying code or otherwise appears as a virus (if it immediately tries to find other executables, for example), one could assume that a virus has infected the executable. However, this method too results in a lot of false positives.

Yet another detection method involves using a sandbox. A sandbox emulates the operating system and runs the executable in this simulation. After the program has terminated, software analyses the sandbox for any changes which might indicate a virus. Because of performance issues, this type of detection normally only takes place during on-demand scans.