"SBS 2003 is perfect for 1 to 75 client workstations."

Sure. Just don't expose that delicate, vulnerable stack to the bad guys on the Internet. You'll be 0wned and spamming, and *you won't even know it*. Hardly anybody (one in half a million, reliable estimate) bothers to report spam any more, and the big ISPs don't even act on complaints.

Now consider, if the bad guy's trojan can send spam, he can just as easily steal your business data.

If you run a Microsoft office, you need a robust bastion host between you and the criminals.
Two, if possible. One to de-fang incoming hostile Web content, and another to proxy and de-fang email.