Why? And is there any other kind?
It is not difficult to change a virus signature to avoid detection. There are actually services designed just for that purpose. They take a piece of malware and scan it to see if it is detectable by common AV programs. If it is, they encrypt it, sometimes more than once, and run the check again. Once it is undetectable, they give it to the customer. It's one of the larger criminal enterprises out there. Signature based detection will always be a step or two behind.
A better solution is to close the holes that malware uses to gain entry. Usually this is done by turning off or removing services and software you don't need, updating your software, and by being a smart user.
edit: Now, I did say "
mostly useless" for a reason. Sig based AV is good for protection against lazy criminals and script kiddies, both of whom will typically not take advantage of the services I mentioned above.
