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, some release one daily, others make a release whenever there is new data. The best policy for updating your anti-virus software is "whenever a new pattern file is available", however, a weekly update supplemented by an emergency download whenever a "Medium" or "High" risk virus is announced is adaquate for many users. Automating the update is highly recommended.
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.