|SBS = small business server.|
You are doing peer to peer networking. A xp pro box can only have up to 10 network connections to it max. this means what is shared on it can only go to 10 others at a time. This also means 20 people won't be able to access the xp pro share.
xp home only supports 5 connections.
Proper configuration should have been a DC with all users files/shares on it and not on the local workstations at all. This allows you to backup everything important when you backup the server.
A pc can not be joined to the domain and a workgroup at the same time. This I suspect is the source of your problem along with the above concerning peer to peer limits.
You have quite a mess with the mixture you have. I would suggest reading a book on 2008 and networking then discuss with management what your best approach to correcting these issues would be. Might even be a good time to bring an experienced network person in to help straighten this out.