It's certainly possible yes, some desktop machines have/had BIOS-based AV but doing it 'in-NIC' would require quite a lot more logic in the NIC, thus costing more, plus a mechanism for them to hold the, usually quite large, virus-definitions on-chip too. Oh and this system wouldn't necessarily be any more secure or faster than doing it 'in-cpu' but would almost certainly slow down the NIC. What many if not all AV products can do is look at traffic coming in via the IP-stack and search for viruses - this is quick and easy to update the defs as it's AV-product specific rather than AV-product-AND-NIC specific.
Hopefully I've clarified why NIC-based AV would be quite a poor idea and why if implemented it would be quite easy for newer viruses to not be spotted by out-of-date NIC-based defs.
By the way, this sounds like a homework question - if it is can you come back to us with the mark you get ;)