Loose rivets,
There are several Viruses that will not only turn off Virus Scanners, but there are Viruses that modify the Virus Scanners to still scan but not find any viruses. That way the users think they are scanning and are safe when they are not.
That is the main reason why I first tell people to run:
Trend Micro's HouseCall
That way we start the troubleshooting with one good scan.
Take Care,
Richard