Does other internet services work, or is it just the browser? Try to ping a web adress. If it works, try another browser
Doesn't work? If you type ifconfig, does the output for eth0 match the settings your isp has given you? If not, see the man page for ifconfig on adjusting the values (gateway, netmask and so on). Well, I've never used a modem... Is your network connection out still called eth0? Perhaps a modem is handled in a completely different way?