Pinging 127.0.0.1 only confirms it can ping itself not dns.
ping tests just confirms it’s able to talk to the other devices on the network. On your XP machine go the command line and type this:
You should get something like:
Default Server: name_dns_server_fully_qualified_domain_name
What is your response?
If you don’t get the message above and something like server or you error out try this
If this works, this means that your dhcp server may not be providing dhcp settings.
Check your dhcp settings and confirm they are provided the local server for DNS settings. Your server should be providing as a dns settings as 192.168.0.100 and not 127.0.0.1 to your clients. On the server itself, it would be pointing to itself via ip 127.0.0.1 or better use 192.168.0.100.