Lets see...
You have a Windows 2000
Server maching running as a workgroup.
The server has clients on it and its a workgroup?
Well, I suggest you change the workgroup to a domain structure and 99% of your XP problems will go away.
I am not sure why but XP's workgroup networking really sucks!
Doublecheck any Firewall settings and make sure all machines have the same workgroup name assigned.