What does antivirus software do?

There are a variety of antivirus software packages that operate in many different ways, depending on how the vendor chose to implement their software. What they have in common, though, is that they all look for patterns in the files or memory of your computer that indicate the possible presence of a known virus. Antivirus packages know what to look for through the use of virus profiles (sometimes called "signatures") provided by the vendor.

New viruses are discovered daily. The effectiveness of antivirus software is dependent on having the latest virus profiles installed on your computer so that it can look for recently discovered viruses. It is important to keep these profiles up to date.

More information about viruses and antivirus software can be found on the CERT Computer Virus Resource page
http://www.cert.org/other_sources/viruses.html

What is NAT?

Network Address Translation (NAT) provides a way to hide the IP addresses of a private network from the Internet while still allowing computers on that network to access the Internet. NAT can be used in many different ways, but one method frequently used by home users is called "masquerading".
Using NAT masquerading, one or more devices on a LAN can be made to appear as a single IP address to the outside Internet. This allows for multiple computers in a home network to use a single cable modem or DSL connection without requiring the ISP to provide more than one IP address to the user. Using this method, the ISP-assigned IP address can be either static or dynamic. Most network firewalls support NAT masquerading.