Question : computers are not able to connect to Internet, but are able to connect to Network

We have 4 computers that are not able to connect to Internet (browse the Net). These computers are able to send and receive e-mail outside to and from outside the network. Iven though these 4 workstations are able to connect to our network cannot browse the Net.
These computers run Windows XP Pro, domain controler Windows server 2003.

Answer : computers are not able to connect to Internet, but are able to connect to Network

Doing a repair install did not fix the problem.

In one of the computers I did a clean install of the O/S - format the HD and all that. Did not fix the problem. So a virus was not the problem.
Then what I did was to connect a 4-port swith the a cable that goes into a good computer then connect the computer with the problem to the switch, there the computer was able to browse the Internet.

Knowing this  my focus was on the routers/switches that we have in the network.

we have several switches/routers in the network (daisy chain style); one of the switches was the problem (16 port switch - Hawkings) there were 3 wires connected (two computers and one for anoter switch - attached to this devices were 4 computers).
All together we had 6 computers with the same problem, two users did not attempt to browse the Net and they did not notice the problem (their e-mail was working fine). Anyways, I replace the switch and everything is ok now.