Can connect to intranet but not internet

We have a machine which can connect to the intranet but not the internet.

I didn't set it up initially but all the Network and Sharing settings are correct as far as I can tell; they're are the same as our other machines (which can connect to the internet).

Firewall is on, as per other machines, and IP details are configured manually, as per other machines, including DNS Servers.

I've tried resetting out ADSL modem and Switch, to no avail. I have also noticed that the fan on our Switch has stopped working so there's a chance that it may be damaged and causing the problem, however I would expect not to be able to connect to the intranet if this was the case.

Any ideas?