TCP/IP connection

I have a small network of PC’s using a DHCP server for addressing. Recently I had the netzky virus on one of them and since cleaning it out have not been able to get that computer on the internet. I can browse the local network with no problems and I can ping the server but not the gateway. I have changed the address to a static one and found I can get on the internet again. What I can’t figure out is why that one particular address didn’t work for the internet. Any ideas out there?