We do not need antivirus if there is no Internet connection.
Internet is not the only source of infection.
Floppy disks, external hard drive, a USB key, an MP3 player or even a single memory card digital camera can be used to infect a computer.
In fact, it is even worse: Even the CD "officials" are not immune. We have already found viruses on CDs of some magazines, not counting some root-kits on audio CD (Sony).
It is therefore essential to have an antivirus, even when there is no Internet connection.
Moreover, it is quite possible to maintain updated anti-virus even without Internet connection.