If the domain doesn't have a dedicated IP here are two options:
1. On Windows XP go to c:\windows\system32\drivers\etc and add an entry in your hosts file for your domain - matching the server's IP address.
eg
203.34.242.10
www.yourdomain.com
2. Using an existing domain on the server, add the following to a vhost.conf file in the /domain/conf folder:
ServerAlias newdomain.com.existingdomain.com
www.newdomain.com.existingdomain.com
and then run /usr/local/psa/admin/sbin/websrvmng --reconfigure-all