Q:

Cannot ping Windows 2008 R2 by hostname

I cannot ping the Windows 2008 R2 server by hostname from Windows XP Home machines, but I can ping it from Windows 7 Ultimate machines.  I already disabled ipv6.  I need to access a shared folder: \server\a
All computers are on the same local network, there is no DNS in use.  Just every computer in the same Workgroup. 
Any ideas why? 
EDITED:
I have heard that the problem could be that different systems chose different Master browsers. 

A:

Judging by your comments that you aren't using DNS, your problem is NetBIOS name resolution. 
NetBIOS over TCP/IP is disabled in Windows Server 2008 R2 by default, however it is the only way for XP to handle local name resolution without a DNS server.  This problem does not exist for newer OSes on your network, because they use LLMNR/WS-Discovery, and not NetBIOS. 
You can enable the NetBIOS responder in the WINS section of the IPv4 network settings:

You also need to make sure that the Computer Browser service is started and that Windows Firewall is allowing inbound IPv4 NetBIOS traffic.