As stated XP (and Windows 2000 Pro) have an upper limit of 10 concurrent connections. You are not running a
domain as you don't have a domain controller (ie a server product), you are running a
workgroup. It doesn't matter how many workgroups you set up or what you call them, you will only get 10 concurrent connections. Therefore, the short answer is yes, buy a Windows 2000 server. This is expensive but there are plenty of plus points: centralised administration (just setup accounts on your new domain rather than going to ech idividual computer), Active Directory (your centralised domain database), group policy objects (setting security privaledges and remotely installing software etc), etc. If you buy SBS 2000 you can have a domain up to 50 clients and you have proxy (internet sharing), SQL and exchange (email solution) built in for a reasonable price. As far as licensing goes, you will need a license for each computer that connects to the server but you can always turn off the licensing service. Oh, and get it installed by a professional such as
http://www.integrationinnovators.com