Cannot browse to some sites via network

We have a small office network that utilises 5 PC’s and every PC has Windows XP Professional. The one PC acts as a gateway and has an ADSL internet connection. The problem is that we cannot browse to certain websites from the computers on the network, but we can browse to any website with the gateway machine. What can be the problem???
I tried pinging the websites in question and the operation timed out all the time