Create a new web site bound to "All Unassigned" IP addresses for ports 80 and 5555 and select the "CRM" directory you referenced as the web site's home directory. Configure a "Host Header" on the port 80 binding for "crm" (and create a host header for "crm.fully-qualified-domain.com" if you want it to be accessible with the FQDN, too). Finally, create a CNAME record in your domain's DNS zone for "crm" referring to the name of the Windows SBS Server computer.
That will preserve the default web site configuration (which you may need for other services-- Remote Web Workplace being the main one I can think of) but give you what you're looking for.
I've got a couple of minutes while I'm waiting on some files to copy so I thought I'd feed my Server Fault addiction... >smile<
This is Microsoft's Dynamics CRM software, eh? (I got that from reading your other questions... I hadn't looked there until this moment). I don't have any experience with this software. I suspect that there's more configuration in the web site that the product's installer creates than just pointing it at a "home directory" and leaving the rest of the IIS settings as-default. A fairly determine searching effort isn't bringing me to any documents that describe what to do if you've deleted this web site by accident (which would be a similar scenario), so I can't say for sure what needs to be done to create the site.
I suspect that IIS 6 may not be setting the proper default document name and there may be some authentication issues, too (like not allowing anonymous access to the default document to force authentication). That's just my gut talking after reading the few things I did.
If this server isn't production for any other role I'd beat on the setup a little more, concentrating in particular on any logs the setup might generate that could give you some clues as to why it's not creating its web site.