In order to get optimum performance from any anti-virus software, users need to Update the Virus Pattern file or Signature file, and Upgrade the anti-virus program accordingly.
Virus patterns are variously called "signatures", or virus definitions, basically they are the data that describes how to recognise the viruses. Anti-virus developers release new virus pattern files frequently, many times a day. The best policy for updating your anti-virus software is "whenever a new pattern file is available", and the software's own automatic update mechanisms are usually the best way of achieving that. However, systems that are isolated from the internet will be unable to download their own updates. In those cases, a weekly update supplemented by an emergency download whenever a "Medium" or "High" risk virus is announced may be adequate.
Like all software developers, anti-virus developers continually improve their products. The improvements may include changes to cope with new operating systems and file formats, or bug fixes. New features may also be added. In some cases, a novel new virus will make developers change the way they look for viruses. Upgrades should be installed when available, as the maintenance contract allows.