Virus scanners work by comparing the data on your computer against a collection of virus "signatures." Each signature is characteristic of a particular virus, and when the scanner finds data in a file, e-mail, or elsewhere that matches the signature, it concludes that it's found a virus. However, a virus scanner can only scan for the viruses it knows about. It's vital that you keep your virus scanner's signature file up-to-date, as new viruses are created every day.
The problem actually goes a bit deeper than this, though. Typically, a new virus will do the greatest amount of damage during the early stages of its life, precisely because few people will be able to detect it. Once word gets around that a new virus is on the loose and people update their virus signatures, the spread of the virus falls off drastically. The key is to get ahead of the curve, and have updated signature files on your computer before the virus hits.
Virtually every maker of antivirus software provides a way to get free updated signature files from their Web site. In fact, many have "push" services, in which they'll send notification every time a new signature file is released. Use these services. Also, keep the virus scanner itselfthat is, the scanning softwareupdated as well. Virus writers periodically develop new techniques that require that the scanners change how they do their work.