Re-installing is the only alternative. You can't downgrade from Enterprise Edition to a lower edition. Practically speaking, the re-install time is trivial compared to the time to configure an office to use SBS.
If you want to maintain the Active Directory structure (accounts, etc.), you could do it with a Swing Migration and a 2nd computer as a temporary second DC, but I doubt it's worth the effort for a 5-person company. If they already have a Domain set up, you may want to migrate User Profiles on the client PCs. Be sure to read up on how to do that BEFORE you actually remove the client PCs from the old Domain.
Dell has a small server with SBS 2003 R2 pre-installed (but still requiring configuration) for $850 (with two SATA 80GB drives in RAID 1 configuration) this week.
Dell SC440 with dual 80GB SATA drives in RAID 1 configuration and SBS 2003 R2 pre-installed.
Actually, it's kewl that Dell has added a "cheap" SATA RAID controller option to the SC4xx models. They dropped it when they discontinued the SC420, making this model virtually a non-contender for even tiny businesses. By the time you added Dell's CERC RAID controller, the price came out the same as the larger PE830 or PE1800 models.
Finally:
If you are going to configure SBS 2003, be sure to get a book on how to do it correctly. If you've never done it before, the odds are pretty high of making mistakes that can reduce its functionality and be costly to fix. It's not "hard" to do, but you need to know the "right" answers to the configuration questions.
Harry Brelsford's SBS 2003 Best Practices book, available for $40 or so on Amazon, is good for a first-time configuration.
