ISA is a software firewall that comes bundled as part of Microsoft Small Business Server Premium. It is an optional component and yet tends to be either loved or hated. If loved and you are installing MS Dynamics CRM SBE in the mix then you need to do a bit of research.
Luckily a few of the tough and passionate SBS Professionals and SBS MVPs have done some of the work for you.
A major thanks to HandyAndy's persistence in mastering MS Dynamics CRM SBE and ISA. His latest article on the subject can be found here
AND before Dana borrows Susan's 2x4 and smacks me.. Please make note.. You need to define, refine and master your own security best practices. One solution is not necessarily the best solution for all environments. The nice thing about MS Dynamics CRM is you DO have choice.