Handy Andy over at SBS Rocks has written a step by step how to titled Install CRM SBE on SBS 2003 Premium with ISA 2004. I have not used it myself but after watching a live install during our last SBS user group meeting and reading Andy's article I now feel ready to give it a go. Check out his article if you'll be installing CRM.
There's couple of tweaks you need to make and the unmake after the installation. Be sure to also read the manual on this one. CRM is going to put its hooks into every nook and cranny of your SBS server and you'll need to be aware of what's going on.