intenet connection problem

My office is using adsl connection which is connected to webramp router and 3com switch. For the last few days the internet connections has been down. I've tried restarting the adsl modem and router but still fail. From client pc i can ping to the router but fail to get reply from the website.
My question would be, what would have cause this?,Could it be because of the viruses and/or hijacks, adware on client PC?. Is there any tools that i could use to monitor irregularities in network traffic to avoid the problem?.

Thank you for all your suggestions and advice.