To start with a provacative answer : I don't think it does on the client. Two recent horror stories:

• Norton anti virus is notorious. I've been doing a lot with COM, a major part of FAQ lists and support was dealing with nav disrupting automation functionality. This weekend I receievd an email from someone appologizing for not having replied before. Norton had been blocking all of his outgoing mail for over a week.
• Someone else had made big investments in virus protection. Installed a lot of software and set up update subscriptions. Monday morning in his office. He switches on his computer, checks his mail, checks for virus-updates.... Bang.... Virus.

The second is imho the keypoint why it makes no sense to have client side virus protection. Your mail provider should do the check. And most of them do. But the time gap between a new virus and an update of the profile at the provider is long enough for a virus to get through. If a big provider, whose is on the allert 24 hours a day, can't even catch all (and I don't blame them), how do you expect to keep up ?

The only thing that does help is a good procedure. The user (and all his employees !) in the second case should have checked for the virus update before checking his mail. I use this procedure

1. Never click an unknown attachment
2. Make sure your machines have an IP address which is by definition unreachable. (192.168.x.x)

I have no virus scanner and neither me or my wife or my kids have (so far) been infected. I even challenge you to try to get through :>

Peter