Here's an effective way to keep GAL-crawling viruses from spreading throughout your company.
Create a fake user with a name that will be listed first, alphabetically in your company's GAL, such as Aabott. Have all e-mail sent to that user go to you -- your pager, your e-mail account, your cell phone, everywhere.
Chances are, if that user is receiving mail, there's a problem because no one should be explicity selecting that user when sending e-mail.
As soon as you see that the message received is a virus, you can open the mailbox properties of the offending user and restrict that user from sending messages out larger than 1k. That will keep the viruses from spreading while you deal with that person's machine.