It is absolutely crucial that every computer that will ever have access to the internet also be outfitted with reliable and up-to-date virus protection. Even if a computer is not connected to the internet, it can still be infected with viruses from other computers it may be connected to, as well as from disks and portable storage devices.
Anti-Virus software can be expensive, but there are also free versions that are just as reliable, such as AVG Free Edition and Avast! Antivirus Trial Edition. Schedule the anti-virus to scan your computer on a daily basis, preferably during a time you are not using it. Again, no matter which anti-virus software you choose, it must be kept up-to-date.