Regardless of size , all networks face pretty much the same threats. The difference with networks in small businesses is that they rarely have someone dedicated to their proper care and feeding. There's a difference between Windows Server 2003 and Small Business Server 2003: SBS includes a number of wizards that, if you follow them, automate much of the work needed to get and stay secure. Spend a little time learning about and configuring the security of your servers and your network can become largely self-maintaining, letting you spend time managing your business instead.
Small businesses aren't large enterprises ; they don't have the luxury of enterprise consultants who can tweak every setting (and maybe that means small businesses are better off, because tweaking can be dangerous).  Yes, with SBS you're running all your roles on one box, but if you don't follow basic security practices then it really doesn't matter how many boxes you have! But with good security practices, such as we describe in this chapter, you can safely combine roles onto a single computer (or maybe two)it's all about balancing cost, time, and security.
 For the enterprise folks reading this chapter, you can learn a lot about the special concerns of small businesses by checking out the thriving Windows Small Business Server Community at http://www.microsoft.com/windowsserver2003/sbs/community/default.mspx.
Microsoft has published several resources useful for security in small businesses. Please spend time with these to make sure you're as secure as you can be:
Security Guidance Center for small businesses
Small business computer security checklist
The e-security guide for small businesses
Securing Your Windows Small Business Server 2003 Network
Securing Your Network: Identifying SMB Network Perimeters
Securing Windows XP Professional Clients in a Windows Server Environment