Eu estou supondo que você está usando endereços IP para ping com, e não o nome do host (se o último, então você tem um problema de resolução de nome de host).
For the former, it is most likely a firewall issue. When Windows firewall is on (as the default settings) in Windows XP computer, it will not respond to ping requests, so obviously you can’t ping to the particular computer. You will be getting ‘Request Time Out’ reply from the destination, even though its connected and accessible in local network. The reason behind is that the ICMP packets, which are used in ping command, are blocked in default firewall rule in Windows XP. The 'pinging' host sends ICMP packets to the destination and waits for the response. When the destination computer doesn’t acknowledge or reply these packets you will be seeing ‘Request time out’ reply.
There are two methods by which you can enable ping, or ICMP Requests, in XP.
1) Completely switching off the firewall service (This is not recommended in most cases)
2) Only enable ICMP packets (excepting ICMP packets in Firewall rule), so while firewall is on ping requests also will work. You can do this thus:
a) Go to Network connections in Control panel;
b) Right Click the connection you want to enable ICMP/Ping requests (normally local network);
c) Click Properties;
d) Go to Advanced tab and press Settings button under Windows Firewall;
e) Press Settings button in ICMP area, and finally;
f) Enable the tick in ‘Allow incoming echo…..’. Press OK.
Now this computer will accept ping ICMP echo requests and reply to them.
By enabling Echo or ICMP or Ping requests by this method without completely switching off the firewall, the computer will be safe in network and reply to ping/ICMP requests. That will allow you to monitor and manage IP address and hosts on your network.
Fonte: Como ativar o ping no XP