If this happens to you the simplest answer is to recreate the web access directories. You can't just delete them and restart the SA service as you did in Exchange 2000!
1. Backup the IIS metabase
2. Delete the Exchange virtual directories in IIS Manager
3. Delete the /AD2MB path using ADSUTIL.VBS
4. Restart Exchange 2003 SA
5. Restore permissions to virtual directories
Microsoft has it covered here.