Apple built-in anti-virus?

If you're running Mac OS X then you already exceed the hardware requirements.
Most of those requirements aren't hard requirements at all - more that if your machine is less than 300Mhz it's likely to be so old and slow you won't reap the benefits of broadband and they don't want you calling their support line complaining that the service is slow when the problem really is that your machine can't keep up with the network.

The Ethernet card issue is simply one of terminology - traditionally, and in many PCs, there is no built-in ethernet and you have to add a card to get the interface. This card is known as a Network Interface Card, hence the acronym NIC. You don't need a NIC since the built-in ethernet port already performs that task.

As for anti-virus, this also stems from the Windows-world where they are endemic to the point where a high proportion of Windows systems on the net are compromised and acting as either spam gateways, proxies, bot servers, or other tasks that consume bandwidth. As a result the ISP is trying to protect their own network investment by encouraging (forcing?) users to use anti-virus software. Since there are 114,000 known viruses for Windows and zero for Mac you could say you're already running the best anti-virus software there is - Mac OS X.

If you really want to follow the letter of the contract and not just the spirit, you can get third-party anti-virus software for your Mac. Check Norton Anti-Virus for Macintosh or VirusBarrier.