Antivirus programs basically search for malicious codes in the system. Most also scan downloaded files and email attachments. Antivirus programs whichever is used must be regularly updated for it to be effective against the latest threats. If the program does not have the latest virus signatures it may not identify the virus  even though most programs come with heuristic algorithms which may detect malicious activities. Keeping an antivirus system updated with the latest virus signatures is the minimum standard that should be practiced. This may come up to at least 1 or 2 updates a week.

Firewalls are programs that control access to and from a system. It decides which program gets to access the outside network and who can access the system from the connected network. When an unauthorized or malicious program tries to send data out of the computer, a well configured firewall will disallow it or prompt the user for instructions. Likewise, when a program from another computer on the network, tries to access data or resources in your system, the firewall will decide if this is a legitimate request or prompt the user. Firewalls need not be updated as often like an antivirus. However, it is important that you get it properly configured at least once when it is installed.