Could Not Communicate With Remote Host

We have 2 computers on one cable connection that use a router. The main computer is running fine with access to the internet. The second computer had been connecting fine but when we went to use it a couple of days ago it doesn't give you access to the web. It uses winxp and accesses via Realtek Ethernet card. I've tried to ping and it shows 4 sent and 4 received. I've tried to repair the connection but it says it cannot renew the address. I've checked the firewalls etc and nothing has been changed on the computer that I'm aware of. The small icon on the bottom shows the comp screens with lights on which looks like its connecting but it doesn't.
I ran PC Doctor and the network test failed and the above message "could not communicate with remote host" was the reason.
Can anyone give me some ideas of where to look?
Thanks:confused